Here is the Collect for 19 December, duing this period of intense preparation before Christmas:
Deus, qui splendorem gloriae tuae
per sacrae Virginis partum mundo dignatus es revelare,
tribue, quaesumus, ut tantae incarnationis mysterium
et fidei integratate colamus,
et devotio semper obsequio frequentemus.
This is from Rotulus 2 published together with the Veronese Sacramentary. It is not in any previous edition of the Missale Romanum.
The vocabulary is loaded here. First, remember that mysterium is interchangable with sacramentum, and it stands not only for our salvation, but also the celebration of Holy Mass, the Eucharist. Frequento is used to describe the participation of Christians in the sacred mysteries.
O God, who deigned to reveal to the world the splendor of Your glory,
through the holy Virgin’s giving birth,
grant, we entreat You, that we both may reverence the mystery of the great incarnation
with integrity of faith,
and we may attend it always with obedient devotion.