Question: I
forgot it was a taanis and made myself a cup of tea. What’s the halacha
now that I’ve broken the taanis?
Answer:
The Shulchan Aruch (OC 568:1) writes that if one accidentally ate on a taanis
tzibbur, communal fast day, one must continue fasting. The Mishna Berura (568:1)
adds that this applies equally to one who purposely ate.
The Rema (Darkei Moshe OC) and Magen Avraham (568:4) quote the
Maharil who instructed one who accidentally ate on asara b’teves to fast
three fasts as an atonement. The Kaf Hachaim (OC 549:7) writes that while this
incident occurred on asara b’teves, this halacha would apply
equally to any other taanis tzibbur. Nonetheless, the Magen Avraham and
Kaf Hachaim quote the Terumas Hadeshen (156) who writes that this is not
required.
Thus, while the Shulchan Aruch writes that one who broke a personal fast
would have to make up for breaking it by fasting on another day, the Mishna
Berura (568:8) writes that this doesn’t apply to one who ate on a taanis
tzibbur.
The Mishna Berura (568:3)
writes that even if one has eaten, they may still say aneinu in mincha.
Yet, elsewhere (Biur Halacha 565:1) he quotes the Chayei Adam who maintains
that one who isn’t fasting should omit aneinu. R’ Shmuel Wosner (Shevet
Halevi 5:60:4; 8:131) explains the difference. One who cannot fast due to ill
health, etc. cannot say aneinu as they are exempt. One who has eaten on
their fast, can say it, however, as they are still obligated to fast.
In conclusion, if one accidentally ate or drank on a taanis, one must continue fasting until
the end of the day. One should still say aneinu in mincha.