There are actually two & they are different nights. Everyone on board is invited to the general reception on formal night. It's kind of like a big commercial for the line with a bit of entertainment thrown in (About 5 minutes). The free drinks are to entice you to come. I was under teh impression you could only have what was being passed around: cheap champagne or teh "drink of the day".
There is a private captain's reception by invitation only usually reserved for VIPs & the people in suites.