Charles Spurgeon Commentary John 16:20-21

"Verily, verily, I say unto you, that ye shall weep and lament, but the world shall rejoice: ye shall be sorrowful, but your sorrow shall be turned into joy. A woman when she is in travail hath sorrow, because her hour is come: but when she is delivered of the child, she remembereth no more the anguish, for the joy that a man is born into the world." — John 16:20-21 (ASV)

And ye shall be sorrowful, but your sorrow shall be turned into joy. A woman when she is in travail hath sorrow, because her hour is come: but as soon as she is delivered of the child, she remembereth no more the anguish, for joy that a man is born into the world.

So, when Christ came back again, they would remember no more the sorrow of their travail hour in which they saw him bound, and spat upon, and taken off to execution, and mocked upon the tree. The joy that would come of it all would obliterate the remembrance of the sorrow.