The question seem to come up often, "Why can't we say Alleluia during Lent?" Often the follow up to that is, "Well, then, why can we say Hosanna?" The issue gets slippery with and educated laity. They get that Sundays are "in" Lent and not "of" Lent - that every Sunday of the Church Year is a resurrection day, a mini-Easter - so why then, especially on Sunday do we give up that ancient word of praise.
According to Stuhlman the Alleluias are omitted because "signs of festivity are customarily omitted."* In the same way the colors are subdued and flowers are replaced by greenery, so too is Alleluia omitted from our worship - a way of highlighting the fast that is at hand. And that, I think, makes sense. Even as Sundays aren't really counted in the days of Lent, still, we highlight in our whole lives, individually and corporately, the activity of self-denial, repentance, and preparation that is happening during the season.
So why do we say Hosanna? This is probably the better question, and one that should be repeated with all of those ancient words that the Church uses freely and often assumes her members understand. Hosanna comes to us from our fore-bearers, the Hebrews. Hoshana, found in the procession of the Feast of the Booths (Psalm 118.25-26) means "save, I pray" or "O save now!"* Our common usage on Palm Sunday is the Greek version that eliminates the second h, giving us "Hosanna."
Jesus, as he entered Jerusalem on a donkey or a colt or (in Matthew's confused case) both, was seen by his followers as the hope of Jerusalem. He was to save them from the boot of Pax Romana. He was the savior of the world, the Son of God, over and above Caesar who claimed those titles.*
Today, as the world remains totally out of whack, we join with our ancestors in the faith, our brothers and sisters today, and those who will come after us and ask again (and again and again), "Save us now, O Lord." Hosanna. Less as a word of praise, though it sounds like that in context, and more as a sign of our hope in the power of God to restore all things to him.
So, my sisters and brothers, shout in expectant joy this Sunday as we once again lift up our voices and say, "Hosanna in the highest!"
* I am indebted to Byron Stuhlman's "Prayer Book Rubrics EXPANDED", "The Westminster Dictionary of Theological Terms", and Borg and Crossan's "The Last Week" for help with this post.