In Romance languages "de" is often used as part of prepositional expressions.
înainte de marți (ro)
prima di martedì (it)
antes del martes (es)
It's probably best just to accept as it is, and not try to analyse it too much. But one meaning of "de" is "of", and we have several prepositional expressions with "of" in English:
in front of the church
instead of me
on top of the hill
What's more, there is some sort of connection in English berween "before" and "in front of"