A cry of blessed desperation

Jesus' lament over Jerusalem (Luke 13:31-35) ends on a note of hope, "And I tell you, you will not see me until you say, 'Blessed is he who comes in the name of the Lord!" This is the cry of "Hosanna!" which the crowds would be shouting upon his entering the city. It is also the "Hosanna!" of the Sanctus et Benedictus, the unending hymn of angels, archangels, and all the company of heaven which worshipers join each time they celebrate the Eucharist. 

It is, ultimately, a cry of victory. The Lord's Anointed One (Messiah) will come to save and deliver his people at last. In the present time, however, it is a cry of desperation, "Save us now, Lord! How long will you delay? Come and rescue your people!" This is why the Pharisees were so upset when the crowds greeted Jesus in this way (Matthew 21:15).

Jesus laments over Jerusalem because the Holy City and its people are about to reject God and his promised Messiah. There can be only one way for Jerusalem to be saved now. Its people must come to a place where they know only desperation, and in that desperation, cry out, "Blessed is he who comes in the name of the Lord!"