Consider the text in Gemara Yoma (81.2):
"Rabbi Chiya Bar Rav M'Diftei taught: It is written 'You shall afflict your souls on the ninth day of the month, at evening.' - Do we then fast on the ninth? Isn't the tenth the fast day? Rather, the verse means to say thus: whomever eats and drinks on the ninth the verse credits as though they fasted on the ninth and the tenth."
Now it appears to me that the explanation is as follows: Firstly, this entire verse is unnecessary, for in the beginning of that topic it is written "However, the tenth day of the seventh month is Yom Kippur, a holy convocation for you, and you shall afflict your soul." Furthermore, do we ever find that when Shabbos or the festivals are mentioned the term 'observe' refers to the day and the following evening? Regarding Shabbos it is written 'Remember the day of Shabbos', which includes the previous night, as the night is attached to the day that follows it. The same is true for all the festivals. Even Yom Kippur itself, here and in the portion of 'Pinchas', is referred to as 'the tenth day of the seventh month', which automatically includes the previous night followed by the day. Why then would the Torah specifically point out the day before, stating that the fast starts in the evening? Moreover, the literal translation of the verse itself does not seem proper, as the 'affliction' is not yet apparent at night, since a meal has been recently eaten before the evening. In the main, any 'afflicting' that would be experienced would occur in the daytime.
Furthermore, this cannot be compared to that which is written in the Torah portion of 'Bo': "In the first month, on the fourteenth day, in the evening, you shall eat Matzah", since there "you shall eat" is written at the end of the verse, which implies that at night Matzah is to be eaten. Here, however, "fast" is written before "evening", which implies that there is some 'affliction' immediately on the ninth day (Tosfos, Gemara Brachos 8.2). Also, the text regarding Matzah is absolutely necessary, since the command to eat Matzah is only on the first night, and not in the day. Therefore, the verse explains that the obligation is only on the fourteenth at evening. By contrast, here the day and night are equal. In fact, the main affliction is actually during the day, as I have written.
The verse relating to the final days of Peasach is also not a problem. Though it states "...until the twenty first day in the evening...", this text was necessary to teach that the festival is until the evening of the twenty first, and the day of the twenty first is not included (Heb. 'Ad v'Lo Ad Bichlal'). Yom Kippur, however, has a verse with content that seems completely unnecessary. On top of that, the verse's text is also not properly understood, as I have previously explained.
It is for these reasons that our Rabbis of blessed memory have expounded the verse in Vayikrah 23.32 as though it had been written "You shall afflict your souls on the ninth of the month - that is, in the evening - and from the evening until the evening of the tenth". It is clear from this construction that a) "in the evening" must have some meaning of its own to add separate from the rest of the sentence, and b) "in the evening" must be referring to the evening at the end of the ninth day. The purpose then is to imply quite the opposite [the contrapositive] : If it is not the evening of the ninth, then you shall not fast. This is a prohibition which is implied from a positive command, which gives the sense of "On the ninth you shall not fast, and I will consider it as though you fasted on the ninth as well. This allows the word "afflict" to have some meaning in the sentence in relation to the ninth, as if to say: "As for me, I consider it as though the ninth and tenth were both fast days".
The explanation of the verse "Afflict your souls on the ninth day of the month in the evening" can then be said to be "Prepare yourselves that you should be able to fast in the evening, from evening to evening, by eating sufficiently on the ninth so that you have strength to fast on the tenth. I will then consider it as two fast days." (Compare with Rashi on this verse)
“It is from the love that the Holy One Blessed be He has for Israel that he decreed upon them to fast, and commanded them to eat and drink before hand, so that they should have the strength to fast.” (Bach)
Both days then have the status of a fast. This is similar to the Mishna in the beginning of the fourth chapter of Tannis, where the men of the "Ma'amad" [the weekly rotating group of citizens who were selected to stand in the Temple on behalf of the nation's communal sacrifices] did not fast on Sunday, so that they should not have to pass from the pleasure and rest of Shabbos directly into strain and fasting (see there). This is because a fast is more difficult to endure directly after a period of pleasure. Since there is a command on the day before Yom Kippur to eat, drink, and feel content the following day's fast will certainly feel more difficult. Therefore it is, in a sense, as though the fast were on the ninth and tenth. Though one's ability to withstand the fast is certainly strengthened and improved by eating beforehand, it is the discomfort experienced that is heightened by the immediate change of state.
Due to the above, it is a Mitzvah to eat and drink on the day before Yom Kippur, and to rejoice in it. Our hearts should trust that the day of Yom Kippur will bestow upon us blessing and good tidings, and that the Holy on Blessed be He will forgive our sins, and seal our year for blessings and good outcomes.
On this day we eat fish and meat just as we do on a Festival. It appears that this Mitzvah to eat is only in the daytime before Yom Kippur, and not the previous night of the ninth, for since the purpose is to create strength to better withstand Yom Kippur, the benefits will only be felt from the immediate day before, and not the previous night. This appears as well to be the intent of Rashi in Gemara Kesuvous 5.1 (his comment beginning with 'Yidche', see there). One should even decrease his Torah study in order to eat on drink on this day (Magen Avraham).
Know that the Rambam, in all of his material on the fast of the tenth, does not mention this law of eating and drinking on the ninth. It would seem that his reason for this is obvious:
In Gemara Yoma we find 'Tanayim' (Rabbis of the Mishna era) arguing over the verse we have been considering, where one derives from it to add hours on to Yom Kippur, for he says: "when the verse states 'You shall afflict...etc...on the ninth day of the month', we would think that the fast starts on the ninth. We therefore learn 'in the evening', to teach that it starts then’." He continues that "if it should start in the evening why does the verse state 'on the ninth'? How are these two words reconciled? By one starting during the day". From here we learn that a certain amount of time of the previous day is added on to Yom Kippur. At this point the Gemara points out that another Tanna has learned this law from a different verse, and then asks how this other Tanna would understand this verse in a manner that was not redundant. The Gemara concludes that he has used this verse as the instruction to eat and drink in order to be able to fast.
Now, the Rambam himself writes in the first chapter of the laws of the fast of the tenth, in the sixth law, that "One must add time from the weekday onto the holy day...etc...as it says, 'You shall afflict your souls on the ninth day of the month, at the evening'. This means to say that one must begin the fast on the ninth while it is yet day... etc". Until here is the quote. Since he is of the position that this verse is expounded for adding on hours, he therefore does not recognize the exposition of this verse for eating and drinking. That is why he left it out of his work.
Now if this is indeed so, why did the Rambam mention the following in the Laws of Vows, law 9:
"If one vowed to fast on Sunday or Monday, and it is a Festival or the day before Yom Kippur - he is required to fast...etc...If Chanukah or Purim happened upon those days, his vow is delayed...etc...since the prohibition to fast on these days is from the Sofrim [the Sages], it needs strengthening."
Until here is the quote. The clear implication here is that there is a Biblical prohibition to fast on the day before Yom Kippur, just like on the Festivals, and it therefore does not need 'strengthening'. The classical commentaries thereon are also particular with these words to the same end. (The Kesef Mishna wonders on what basis the Rambam concluded that this is a legally binding Biblical exposition. As far as he can discern, it is merely an 'Asmachta' [a Rabbinic law that indirectly relies on a hint or leading statement found in the Torah]. The Lechem Mishna writes that this is, in fact, a Biblical exposition, which he asserts from a Gemara Rosh Hashana (9.1), as well as Gemara Yoma, which we have mentioned. The Magid Mishna adopts this position as well in the Laws of the Fast of Tenth, see there.)
I was quite surprised by this. If it would enter your mind to think that the Rambam's prohibition against fasting on the day before Yom Kippur is Biblical, or even Rabbinic, obligating one to eat and drink, why would he not list that in his compilation of the Laws of Yom Kippur instead of relying on a indirect mention in the Laws of Vows, where it's purpose is only to be overturned, forcing fasting?
Therefore, it appears in my humble opinion that the clear solution is as I have written, that the Rambam has rejected this law, for the reason I have noted. The fact that in the Law of Vows it is written that one must fast should not be taken to mean that eating on this day is a obligation similar to that of a festival, and is enjoined Biblically or at least via 'Asmachta'. This is not an item that requires 'strengthening'. It is no obligation at all, neither Biblical nor Rabbinic. That's why the vow takes effect.
The reason why it would be deemed necessary to mention this at all is because of the prevalent custom among people to increase their meals on the day before Yom Kippur. This attitude is supported by the Midrash, which relates a story about a tailor who spent lavishly on fish for the day before Yom Kippur, as is brought in the Tur. At the end of the fifth chapter of Chulin (83.1) we similarly find that in the Galil they increased their meals on that day. To prevent people from thinking that this is the result of any command, even Rabbinic, whereby they might conclude that fasting would be prohibited as on Chanuka and Purim, he teaches there is not even a Rabbinical command at stake, and one is required to fast. Moreover, the Rambam is not alone in his opinion, for also in the Rif and Smag I have not found such a law, that there is a command to eat on this day. Their reason is also due to the fact that they use the source verse to teach the need to add time on to the fast (see there).
However, the Rosh, Tur, and Shulchan Aruch do rule as Chiya, that there is a command to eat on the day before Yom Kippur, and it is possible that they believe this to be Biblical, with the 'added time' derived from another verse, through the 'Gezeira Shava' [derivation based on the dual use of a similar, technically unnecessary root word] of 'Etzem' / 'Etzem, as is spoken about there in Gemara Yoma. It is also possible that they are following the position of Rabbi Akiva in Rosh Hashana (9.1), which is based on the laws of the crops of 'Shemitah' [the sabbatical year] (see there). Another alternative is that they may understand that the real meaning of the verse is to instruct the need to add time on to the day, and this 'command' of Chiya's is just connected via an 'Asmachta' (as the Kesef Misha writes in the Laws of Vows).
In addition, from the end of the fifth chapter in Chulin there is a proof for the Rif, Rambam, and Smag. Since it states that in the Galil they increased their meals it implies that in other locales they did not believe this to be a Mitzvah. However, it is true that Tosfos avoids that implications by explaining that people enlarged their meals in all places, though the Galil was especially noted for their eating of fish and meat on that day (see there). This does follow the text of the Gemara in Kesuvous (5.1), which states that in all locales they increased their meals, just as the custom is in our communities today.
Our teacher the Rema writes "that it is prohibited to fast on this day, even over a disturbing dream. As to what would be the law if a vow was made to fast, see chapter 570."
Until here is the quote. One should not wonder how it is possible that this day should be stricter that Shabbos or a Festival, where one may fast over a dream, for he has quite a different view on this matter. Since eating is 'considered' as fasting, as though he fasted on the nineth and tenth, as we have previously written, he has actually 'fasted' (Taz, comment 2, and this would answer the question of the Magen Avraham in comment 1 as well).
Nevertheless, if one wishes to fast until the 'Seudah HaMafshekes' (the last meal before Yom Kippur) he may, as long as he eats this meal. This is due to the fact that there is no set amount of eating necessary to fulfill the obligation to eat (Magen Avraham). In this way he can address the need to act on a disturbing dream as well. [IS THIS CORRECT?]
If one made a vow that he will not eat meat except on Shabbos and the festivals he is able to eat it on the day before Yom Kippur, since people call that day a festival. This, though, only applies to the day and not to the night, for there is no Mitzvah to eat at night, as I have written. There is one who writes that those who have the custom not to eat meat except on the days when 'Tachanun' is not recited may eat meat on this night (Magen Avraham in the name of the Shelah). Even if 'Tachanun' was said in the Mincha preceding that night (Magen Avraham), that night is nevertheless considered 'like a day' when 'Tachanun' is not said.
This is no 'falling on the face' on the day before Yom Kippur. 'Lamnatzeach Y'ancha' is not recited, and 'Mizmor L'Sodah' is also omitted (both in the morning prayers), as the offerings referred to were not brought in Temple times, since there was little time in the day to eat them. [IS THIS CORRECT?]
Furthermore, extended amounts of 'Slichos' are not said before daybreak. Though there are places that do have the custom to recite extended 'Slichos', we do not have that custom. Rather, we start from 'To you, G-d, is righteousness' ('L'cha Hashem Tzedakah') and continue until 'He who hears prayers' ('Shomea Tefillah'), after which we recite two 'Slichos' sections, including the liturgical poem 'Accept the fast of your people' ('Y'ratzeh Tzom Amecha'). We then continue with 'Hear our Voices' ('Shema Kolainu'), confession of sins ('Viduy'), and continue on to 'mercy and forgiveness' ('Rachamim v'Haslichos'). After this we go no further. Though this is our custom, each community follows it's own tradition.
In regard to 'Our Father, Our King' ('Aveinu Malkainu'), there are differing opinions. Our custom is not to say it unless Yom Kippur occurs on Shabbos. Since it is not recited on a Shabbos, we recite it on the day before Yom Kippur in the prayers of the morning, though not in those of the afternoon, as is the practice on Shabbos eve.