Good Pharisees fasted on Mondays and Thursdays. This was not required by the Law, but it was their custom. The followers of John must have had a similar practice.
We are not talking about a national day of fasting, required by the Law, or about private times of fasting for various reasons. We are talking about customs, not law.
Fasting was a tradition related to mourning. Specifically, the Jews of the first century were mourning while waiting for God to rescue them. Even though the exile was technically over, it wasn’t really over. They were waiting for a real reestablishment of Israel as a real, independent nation.
Jesus says it isn’t time for mourning any more. He has come. He is the bridegroom. There will be times for mourning and fasting, but not now.
The two homespun pictures of cloth to patch a cloak and wine in wineskins are powerful images. The trick is in how to apply the images.
It seems to me he is announcing a new regime. The old customs and traditions won’t fit any more.
It’s time for a party, not for a funeral.