There's one 747 and one 777 every day and they always take the same legs:
747 Day-flight out, night flight back
777 Night flight out, day flight back.

If you're flying overnight on the way out and during the day on the way back then you will have got what you booked, and probably confused it with the other option.

If not then there must be an unusual substitution for that day alone, but it seems unlikely this far out.