Is it OK to ask “Can I...”?
'Mayn’t' Isn’t OK
Before we answer Donna’s question, let’s talk about denying permission: No, you may not turn off your listening device just yet. It’s possible to say the obscure contraction “mayn’t,” but I wouldn’t recommend it. The American Heritage Dictionary says mayn’t sounds unnatural, and Garner’s Modern English Usage states that “educated people” typically say, “Can’t I?” instead of “Mayn’t I?” or “May I not?” So if we were in the land of strict grammar rules, we might hear Miss Fuzzywink asking her governess, “But why can’t I go to the ball?” Even she probably wouldn’t say, “Why mayn’t I?”
'Can' Versus 'May'
Now we can ponder Donna’s question about “Can or may we expect you tomorrow?” First, we might ask ourselves if the speaker is talking about ability or permission. I don’t think we're talking about permission: “Are we allowed to expect you tomorrow?” No.
Neither does it seem to be talking about ability—“Are we mentally able to expect you tomorrow?”—at least, not in this literal sense. But it does work in the sense of "Will we be able to expect you tomorrow?" In other words, can we make plans to have you at our house?
"Can" also has meanings beyond simple ability. For example, it can also mean to have the possibility of something, such as "I hear you can win at poker." Therefore, "Can we expect you tomorrow?" could also have the sense of "Is there a possibility that you will arrive tomorrow?"
If we want to ask, “Are you coming tomorrow?” maybe we should just say it that way. But if I were forced to choose between “can” and “may,” I would say, “Can we expect you tomorrow?”
1. Garner, B. Garner's Modern English Usage. Oxford: Oxford University Press. 2016, p. 139-40.
2. Burchfield, R. W, ed. The New Fowler’s Modern English Usage. Third edition. New York: Oxford, 1996, p. 126.
3. American Heritage Guide to Contemporary Usage and Style. Boston: Houghton Mifflin Company, 2005, p. 74.