Jesus tells the scribes that it would be unreasonable to ask his family not to partake in a wedding feast on the wedding day. He then adds a disturbing note, pronouncing that after he was gone, there would, indeed, be fasting.
When the Flavians took the Roman legions through Galilee and Judea, they laid siege on the recalcitrant cities, allowing disease and starvation to do most of the work. The army would encircle a city, trapping the inhabitants and preventing support from reaching them. The agony of starvation was enhanced with humiliation of cannibalism: there were many stories of families eating the remains of their dead, or killing their children for food. When the cities finally relented, Romans would torment the survivors with rape, slavery, and crucifixion before smashing the walls and burning down the city.
Right there in verse 20, Jesus predicts that when Rome removes its grace and removes the Jews from this land, there will be a lot of not eating. This is such a harsh dig on the Pharisees: “You want to fast? There will be such fasting! Just you wait.”