John Gill Commentary Luke 5:35

"But the days will come; and when the bridegroom shall be taken away from them, then will they fast in those days." — Luke 5:35 (ASV)

But the days will come
And that in a very little time, as they did:

when the bridegroom shall be taken away from them :
as their master, John, was taken away from them, and now in prison, and therefore it was no wonder they mourned and fasted; signifying, that in a short time he, the bridegroom of his church and people, should be taken away by death:

and then they shall fast in those days ;
mourn, and be humbled, of which fasting was, a sign, for the death of their Lord, and on account of the many afflictions and persecutions they should endure for his sake; (See Gill on Matthew 9:15).