A person who wishes to observe a Ta'anit Yahid, a personal fast day, must verbally accept upon himself the fast during Minha on the day prior to the fast; otherwise, his abstention from food and drink does not constitute a formal "fast." The Ben Ish Hai (Rabbi Yosef Haim of Baghdad, 1833-1909), in Parashat Re'eh (Shana 2, Halacha 37), addresses the situation of one who forgot who to accept the fast during Minha, and remembers only during Ben Ha'shemashot (the 15-minute period after sunset). In such a case, the Ben Ish Hai writes (listen to audio recording for precise citation), the individual may nevertheless accept upon himself the fast, so long as the Ben Ha'shemashot period did not end.
If a person accepted upon himself a Ta'anit Yahid, may he arise before Alot Ha'shahar (daybreak) on the day of the fast to eat or drink?
The Ben Ish Hai writes that one may drink before daybreak on the day of the fast, and, strictly speaking, he does not even need to make a stipulation to this effect when he goes to sleep the night before. Nevertheless, he adds, it is preferable to verbally stipulate before going to sleep that he intends on drinking before dawn the next morning.
With regard to eating before Alot Ha'shahar, however, the Ben Ish Hai rules that this is forbidden even if one made a verbal stipulation before going to sleep. This ruling stems from the position of the Kabbala that once a person goes to sleep for the night, he may not eat again before reciting Shaharit the next morning. Hence, even if a person stipulates before going to sleep that he intends to eat in the morning, he still may not eat, due to this general prohibition that forbids eating before reciting Shaharit. This is also the view taken by Rabbi Yaakov Haim Sofer (1870-1939) in his Kaf Ha'haim (564:7).
Hacham Ovadia Yosef, however, in Yabia Omer (vol. 5, Orah Haim 22), writes that according to the strict Halacha, one may eat before daybreak on the day of a fast if he made a stipulation to this effect before he went to sleep the previous night. Although according to Kabbalistic tradition it is forbidden to eat after one goes to sleep for the night, this is permissible according to the straightforward Halacha.
Summary: One who wishes to observe a personal fast must verbally accept the fast during Minha in the afternoon before the fast. If he forgot to accept the fast during Minha, he may still do so, even if he remembered only during Ben Ha'shemashot (the 15-minute period after sundown). One may drink before daybreak on the morning of the fast, though preferably he should verbally state before going to sleep that he intends to drink early in the morning before daybreak. According to the straightforward Halacha one may also eat before daybreak if he made a stipulation to this effect the previous night, though according to Kabbalistic teaching one may not eat in the morning.
