What does '48 hours before departure' mean?

I was advised that I have to confirm my flight 48 hours before departure. What does that mean exactly? '48 hours before the departure' is just a "point" of time, not a period. I don't think it means I have to do so 'exactly' 48 hours before the departure because it's almost impossible.

The possibilities are 1) Before '48 hours before the departure' 2) After '48 hours before the departure' 3) Around '48 hours before the departure'

I'm really confused