Acetal formation.

Acid catalysis occurs twice in acetal formation. First, the carbonyl group is activated for nucleophilic attack by protonation, and, later, the hydroxyl group of the tetrahedral intermediate is protonated, making it a better leaving group.

This is the kind of little fact that isn't so much in itself, but is really helpful for remembering the mechanism. The more networked your knowledge is with associations, the more you will retain the fundamentals.