1,357. Fasting on Special Days

Hilchos Taaniyos 1:6

Communal fasts are not to be convened on Shabbos or yom tov. On these days, shofars and trumpets are not sounded and we don’t cry out to God. Exceptions are only made if a city is under siege or being flooded, or if a ship is sinking. In these cases, and even if a single person is being pursued by enemies, bandits or an “evil spirit,” then we may cry out to God for mercy on their behalf. The trumpets are not sounded in these cases unless they are necessary to gather the people in an effort to save them. Hilchos Taaniyos 1:7 Similarly, a fast should optimally not be called for Rosh Chodesh, Chanukah, Purim or chol hamoed. If the community has already started a series of fasts because of some trouble - even if only for a single day - and the fasting pattern calls for a fast on one of these days, then we should fast for that entire day.