1,356. The Rabbinic Obligation to Fast

Hilchos Taaniyos 1:4 In addition to the aforementioned Biblical obligation to cry out to God in times of trouble, there is a rabbinic obligation to fast in times of communal trouble until God’s mercy appears. On these fast days, we cry out, pray and sound just the trumpets; in the Temple, both trumpets and the shofar were sounded. The shofar blasts were shortened and those of the trumpet were lengthened because the mitzvah is meant to be performed using trumpets. The trumpets were only sounded with the shofar in the Temple, as per Psalms 98:6, “Sound trumpets and blow the shofar before Hashem, the King.” Hilchos Taaniyos 1:5 The fasts convened on account of communal troubles are not to be held on consecutive days as this would be too difficult for the people to observe. Communal fasts should be only be held on a Monday, the following Thursday and the subsequent Monday. The pattern of Monday, Thursday, Monday should be repeated until God shows His mercy.