Zechariah — Chapter 8

Haec dicit Dominus exercituum ieiunium quarti

The four fasts (of the fourth, fifth, seventh, and tenth months) shall be 'joy and gladness and cheerful solemnities to the house of Juda.' À Lapide applies this to the Church's transformation of mourning into celebration: the fasts commemorating Jerusalem's fall are replaced by the feasts celebrating Christ's resurrection. Sorrow becomes joy when the cause of sorrow (sin) is removed.