When someone makes a proposal that doesn't work for you, keep the conversation and the relationship going by proposing alternatives. Here's how.
When you're scheduling meetings, it's easy to get caught in an endless back-and-forth as everyone searches for the right amount of time on their calendar. When Bernice emails you, saying "I'm available tomorrow at 3pm. Let's go wild over a box of cupcakes and the latest sales figures," you might discover you're already spending tomorrow at 3pm in a session to decide how to display your latest merchandise. You just can't make it to Bernice's meeting.
Don't simply write back, "I can't make it." If you send just that back, now you've put the other person in a position of still not knowing what a good time would be to meet. Instead, look at your calendar and find 2-3 time slots that work for you. List all of them in your reply, and highlight one as the default. That's the one you put in your calendar. Your reply will look something like this:
"I can't make it Tuesday at noon. My first choice is Friday at 3pm—I'll pencil that in. If that doesn't work, however, I'm free Wednesday at 11am and Thursday 2-4pm. But please let me know quickly, as those time slots may fill up."
This way, you're keep the scheduling conversation going while meeting your scheduling needs and the need of the other person to coordinate a group.