Sent Saturday evening at about 11 p.m.
I'm a 3rd-year student in [Unrelated Department] and would like you to participate in our research project about [unrelated subject]. We would like you to participate on [day this week when I'm not on campus].
Believing firmly in helping students with this kind of thing, I'm usually a sucker for this kind of "interview a professor for a class," "fill out this survey" kind of academic citizenship, but on short notice and a day I'm not on campus? I sent a polite note declining the invitation but refrained from observing that hailing a professor that you've never even met by her first name might not make the best impression. (First names for professors are the norm for grad students but not undergrads around here.)