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?