Here is the Collect for 21 December.
Preces populi tui, quaesumus, Domine, clementer exaudi,
ut, qui de Unigeniti tui in nostra carne adventu laetantur,
cum venerit in sua maiestate,
aeternae vitae praemium consequantur.
This prayer is pretty much identical to a Post Communion in the ancient Gelasian Sacramentary during the "tenth month" ("Decem"-ber). Remember that laetor is deponent.
Gratiously hear the prayers of Your people, we beseech You, O Lord,
so that those who are rejoicing about the Coming of Your Only-Begotten in our flesh,
may attain the reward of eternal life
when He will have come in His majesty.
As we have seen many times, the prayers of Advent look in two directions, back to the historic moment of the Nativity of the Lord and also forward to the moment when He will come in glory to judge the living and the dead.
The prayer juxtaposes caro ("flesh") and maiestas ("glory" or "majesty"). The maiestas here refers to the characteristic of God we see at times revealed in Scripture as, for example, when Moses encounters God in the cloud on the mountain or in the tent of the ark. The encounter with God’s majestic glory (Greek doxa, Hebrew kabod) transforms Moses flesh so that it is so bright that he must wear a veil over his face. The Lord, when He comes, will transform everything in His presence and our sight of Him in the bosom of the Trinity in the Beatific Vision will transform our human flesh forever.
The prayer is also careful to link joy with prayer, as if prayer would be a sine qua non for joy.
>Rhetorial question alert:
Can someone who does not pray truly be happy?
ANOTHER POSSIBLE VERSION:
Listen with clemency, we pray, O Lord,
to the prayers of your people,
that those who rejoice at the coming
of your Only-begotten Son in our flesh
may, when he comes in his glory,
receive the reward of eternal life.