Zechariah 8:19 makes a radical declaration: the fasts of mourning shall become "joy and gladness, and cheerful feasts." The Old Covenant was an era of mourning over the failures of the fathers and the destruction of the Temple. The New Covenant is an era of celebrating the victory of the Son.
When the disciples of John asked Jesus why His disciples did not fast, He replied, "Can the friends of the bridegroom mourn as long as the bridegroom is with them?" (Matthew 9:15). The "feasting" of Zechariah 8 is the spiritual joy of the Church. We do not fast to mourn a destroyed Temple; we feast to celebrate a risen Savior and a completed Kingdom.