How often do you leave your kids with a babysitter? How old are they?

I've never left my children with a sitter before. They are 4, 3 and 18 months. We're leaving them with a sitter this Saturday night (6 - 7 hours...5 of which they *should* be asleep) and I was just wondering...are they still too young to be left with a sitter, say, on a Saturday night while my husband and I go out to dinner and a movie? Or for a few hours in the afternoon so I can go out to lunch? What do you think? And how often do you leave your kids with a sitter/for what reason? How old are your kids?