5. Maariv:

All fasts conclude after nightfall.[1] This is defined as the sighting of three medium stars, or when the moon shines very brightly on the earth.[2] [In all cases, one may begin Maariv before the above time, in order to conclude Maariv and be able to eat when the above time arrives.[3] In such a case, one is to make sure to read Kerias Shema again after nightfall.]

Washing hands after Maariv:[4] Immediately after Maariv, one is to wash his hands three times as is done in the morning upon awakening. A blessing is not recited during this washing. [This washing is to be done also by the Kohanim who already fully washed their hands prior to Nesias Kapayim.[5]]

Q&A May one eat prior to Maariv once the fast is over?[6] Yes. However, if one is able, it is best to delay eating until after Maariv   Organizing a Siyum Misechta and festive meal:[7] One is to make a Siyum Mesechta on Motzei Tisha B’av in addition to the Siyum made on Tisha B’av.

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[1] Michaber 562:1; Rosh; Rav Chisda in Taanis 12 “Whoever did not fast until sunset is not considered to have fasted.” Igros Moshe 4:10 rules that in New York one is to wait until 50 minutes after sunset, and at the very least wait 41 minutes.

Other opinions: Some Poskim rule it one is only required to fast until the start of Bein Hashmashos. [Sefas Emes Taanis 12; Aruch Hashulchan 562:9 based on Rambam, Rashi, Ran] This however is in reference to Zman Rabbienu Tam, and not to the Zman of the Geonim. [Birur Halacha Tinyana 562; Piskeiy Teshuvos 562:1]

[2] Rama ibid; Hagahos Ashri; Hagahos Maimanis

[3] Mateh Efraim 602:29; Igros Moshe ibid; Piskeiy Teshuvos 562:1 footnote 2; Rabbi Groner that so was directive of Rebbe to ask Rav Shmuel Levitan one year on a fast day to Daven before the time of nightfall and announce to everyone to say Shema again after nightfall; See Hiskashrus 1198

[4] Sefer Haminhagim p. 97 [English]; Igros Kodesh 20:284; Ben Ish Chaiy Devarim 28

[5] Likkutei Sichos 9 p. 136 brought in Shaareiy Halacha Uminhag 2:260

The reason: As the impurity spreads with the leave of Tisha B’av. [ibid]

[6] Nitei Gavriel 86:4

[7] Hisvadyus 1989 4 p. 94