This is the 41st most frequent French word.
avant
"Avant" means "before" in English.
Here, 'avant' is used as a preposition to indicate the time preceding an event, which is the dinner in this case.
Je suis arrivé avant le dîner.
I arrived before dinner.
Here, 'avant' modifies 'jour' to form a specific phrase meaning 'the day before', relating to a point in time preceding a referenced day.
Le jour d'avant, il avait oublié ce document.
The day before, he had forgotten this document.
In this example, 'avant' functions as a preposition to indicate temporal order precedence of one person over another.