Do parts of a minor fast start before morning?

Question

I've always understood that minor fasts run from morning (a bit before dawn) until nightfall. An answer to What activities (other than eating and drinking) are forbidden on minor fasts?, however, cites the Mishnah Berurah as saying:

[A] pious person should be stringent on all of them as on Tish'ah Be'Av, but if one of them falls on the evening when [a man's wife is due to] immerse, he should fulfill his obligation.

I wondered how this could even be an issue -- a woman immerses at night, so by the time she's home from the mikvah the fast would surely be over. Joel K there speculated that the MB might be talking about the night before (e.g. the night between 9 Tevet and 10 Tevet). I also note that the MB is talking about a pious person, not necessarily halacha.

This all leaves me confused about pre-fast activities. According to halacha, which if any fast-day changes begin before the start of the fast in the morning?

Answer 1

1. As alluded to by Double AA in a comment on the question, the fast really begins when one retires for the night. Thus, when one wakes up, even if this before dawn, he may no longer eat (unless he stipulated otherwise before going to sleep). See Shulchan Aruch Orach Chayyim 564.

2. As noted by Heshy in a comment on the question, Teimanim and some Sepharadim say Aneinu at ma'ariv on the night preceding a fast day. See Peninei Halachah here.

Answer 2

I had to read MB commentary in O.C. 550:2 a few times. In it he says that a pious person should enact the 5 restrictions which includes marital relations just like it's done on Tish'a B'Av - meaning, that it starts at the night time (tzet of the beginning of say 10 Tevet.) So, he would fast as well as keep the other 4 restrictions from nightfall - all as if it were Tish'a B'Av.

Thus, when it says that a woman should immerse at night and the husband should have marital relations with his wife if this her mikvah night, it means the night of the beginning of say, 10 Tevet. It seems that this situation is the exception for the pious people, but they would still be fasting and enacting the other 4 restrictions at night.