The fast will become a feast – Tishah B’Av
The Bible and the rabbis both accept that in time to come some occasions will assume a different shape.
This is not to be taken as an argument for a reform of Jewish belief and practice.
In the days of the Mashi’ach the content of Judaism will remain, though the historical context will change, turning weeping into rejoicing. When that occurs, the fast of Tishah B’Av – for example – will be a yom-tov.
Zechariah says (8:19): “The fast of the fourth month (Tammuz), of the fifth (Av), the seventh (Tishri), and the tenth (Tevet), shall be to the House of Judah joy and gladness”.