A: We're gonna have a meeting for Monday. It's not correct grammar to use "for" in this sentence. However, you can use "for" in these sentences: "We've schedule the meeting for Monday" OR "We've rescheduled the meeting for Monday." B: I can't attend it on Monday, please reschedule it for when I'm back! This is correct. If you say, "I can't attend it on Monday, please reschedule it on Monday," it would be redundant since you already said "on Monday" at the beginning of the sentence. Also, it means that you are telling someone to wait until Monday before beginning the rescheduling process and we don't know what day the new meeting date will be. Saying "for Monday" makes it clear. "I can't attend it on Monday, please reschedule it on when I'm back," doesn't make sense in English. https://www.thoughtco.com/time-and-place-at-in-on-and-to-4056544 http://www.talkenglish.com/grammar/prepositions-of-to-for.aspx

Did you see "We'll have a meeting for Monday?" in a textbook or on a test? Because if so, I’m surprised they used “for” as an example. It technically isn’t correct to say it that way. You’re right it should be “We’ll have a meeting on Monday.” And it would make sense to say “I can’t attend on Monday, please reschedule it for when I’m back instead.” I think this conversation is an informal one. See, the thing about English is that when we talk, we don’t follow a lot of the proper rules of grammar. You’re completely right with how you think they should be saying the conversation. Your interpretation is just the more polite way and the “correct” answer is more of the quick conversational way that people typically speak (except I think the use of for in “we’ll have a meeting for Monday” is awkward I think that’s a bad example for the writers to use).