air travel – 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