Laws of Rosh Hashanah. Erev Rosh Hashanah (the Fast of Erev Rosh Hashanah)

Duration of the Fast
a. It is customary to fast on Erev Rosh Hashanah, but one should not complete the fast and continue fasting until nightfall. [21]
Regarding the duration of the fast, there are various customs:
a. Among Sephardim, there are those who have the custom to fast the entire day. [22]
b. Among Ashkenazim, there are those who fast until plag haminchah, [23] and they should pray Minchah before eating and recite “Aneinu,” omitting the words “on this day of our fast.” [24]
c. There are those who fast only until the time of minchah gedolah; they should pray Minchah and recite “Aneinu” as above, and afterward they may eat. [25]
d. There are those who are not particular to pray Minchah before eating, and do not wait until the time of minchah gedolah, but wait only until midday and then eat. [26]

Accepting the Fast
b. This fast does not need to be accepted during Minchah of the preceding day. However, one who accepted it upon himself without specifying any limitation must fast until the appearance of the stars. Therefore, one who is concerned that nowadays not everyone has the custom to fast, and that for this reason an acceptance in prayer is required, should state explicitly until what time he is accepting the fast. [27]

Rinsing the Mouth
On this fast it is permitted to rinse one’s mouth, but care must be taken not to swallow water.

A Seudat Mitzvah
d. If a seudat mitzvah occurs for him [that is, a meal for a brit milah or for pidyon haben, and some are lenient even regarding a siyum masechet meal], many authorities hold that he does not need to fast. [28]

Participants in a Siyum Masechet Meal
e. At a siyum masechet meal, if he heard the siyum but did not eat at the siyum meal, it appears that one may be lenient and consider him exempt from the fast. [29]

He Did Not Hear the Siyum
f. If he did not hear the completion of the tractate and came in the middle of the siyum meal, it appears that one may be lenient and permit him to eat. [30]

Leftovers from the Meal
g. One who did not participate in the seudat mitzvah, but food from the seudat mitzvah was sent to his home, may not eat. [31]

One for Whom Fasting Is Difficult
h. One for whom the fast is difficult does not need to fast; therefore, women are lenient in this matter. [32]

[21] Shulchan Aruch (siman 581, se’if 2), and Mishnah Berurah (se’if katan 16, and siman 562, se’if katan 9).
[22] Magen Avraham (siman 562, se’if katan 4) and Mateh Ephraim (siman 581, se’if 36), and see Sha’ar HaTziyun (there, se’if katan 8).
[23] Shulchan Aruch (siman 562, se’if 2) and Mishnah Berurah (se’if katan 10).
[24] Machatzit HaShekel (siman 581, se’if katan 10) and Pri Megadim (there).
[25] Mishnah Berurah (siman 562, se’if katan 10).
[26] See Mishnah Berurah (siman 562, se’if katan 6).
[27] Mishnah Berurah (siman 562, se’if katan 9) and Biur Halachah (there, se’if 2, s.v. keivan), and see Rema (there, se’if 1), Mishnah Berurah (se’if katan 6), and Biur Halachah (s.v. ad).
[28] Rema (siman 581, se’if 2, and siman 568, se’if 2) and Mishnah Berurah there.
[29] See Mishnah Berurah (siman 568, se’if katan 18).
[30] For he is participating in a seudat mitzvah.
[31] Rema (siman 568, there).
[32] Mishnah Berurah (siman 581, se’if katan 19).