S.F.
I went away when my eldest was about three, with his favorite babysitter (and her sister) to stay over and watch. My son had so much fun he didn't even notice I was gone. I called to say hi and he couldn't give me the time of day! When I got back, he realized it meant his beloved sitter was leaving and he started to cry. Too funny...
Anyway, keep it simple. Just say mommy and daddy are going away overnight but you'll be back in two nights. Tell her how much fun she'll have with teacher and she can tell you all about it when you get back. Don't let your emotions about leaving color your words or your tone; she'll pick up her cues from you. Keep it light and airy and don't overplay it. Call if it's convenient. If she REALLY needs to talk to you, the sitter will call you.
Trust me, this is a gazillion times harder for you than for her. With the right set up, it's likely your daughter will be having too much fun to miss you...and that's as it should be!
Good luck.