This Halacha is an excerpt from our Sefer
All fasts conclude after nightfall. This is defined as the sighting of three medium stars, or when the moon shines very brightly on the earth. [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. In such a case, one is to make sure to read Kerias Shema again after nightfall.]
Washing hands after Maariv: 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.]
May one eat prior to Maariv once the fast is over?
Yes. However, if one is able, it is best to delay eating until after Maariv
Organizing a Siyum Misechta and festive meal:
One is to make a Siyum Mesechta on Motzei Tisha B’av in addition to the Siyum made on Tisha B’av.
 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]
 Rama ibid; Hagahos Ashri; Hagahos Maimanis
 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
 Sefer Haminhagim p. 97 [English]; Igros Kodesh 20:284; Ben Ish Chaiy Devarim 28
 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]
 Nitei Gavriel 86:4
 Hisvadyus 1989 4 p. 94