adventus

Adventus, adventus, m., fourth declension, from advenio. It is the standard prose noun for somebody turning up, and the bare ablative does duty as a time clause: "adventu Caesaris" means on Caesar's arrival, which is how Caesar himself writes it. Fourth declension, so genitive singular and nominative plural are both adventus.

Church Latin gives it the capital letter. Adventus Domini is the coming of Christ, both the first and the second, and from there the four weeks of Advent take their name. The corpus writes the forms as aduentu and aduentum because medieval scribes made no distinction between u and v; it is the same word, and a manuscript that writes uenit for venit is doing nothing unusual.