LUKE 5 33 Then they said to Him, “Why do the disciples of John fast often and make prayers, and likewise those of the Pharisees, but Yours eat and drink?”34 And He said to them, “Can you make the friends of the bridegroom fast while the bridegroom is with them? 35 But the days will come when the bridegroom will be taken away from them; then they will fast in those days.”36 Then He spoke a parable to them: “No one puts a piece from a new garment on an old one; otherwise the new makes a tear, and also the piece that was taken out of the new does not match the old. 37 And no one puts new wine into old wineskins; or else the new wine will burst the wineskins and be spilled, and the wineskins will be ruined. 38 But new wine must be put into new wineskins, and both are preserved. 39 And no one, having drunk old wine, immediately desires new; for he says, ‘The old is better.’”[

For as long as the Bridegroom is with us we both rejoice, and can neither fast nor mourn. But when He has gone away through our sins, then a fast must be declared and mourning be enjoined.

(St Bede)