We can't manage the others reaction

Today I adjusted the schedule of client mtg. It's the regular meeting, but the day I planed is the start of system release prepalation. Because I thought it's difficult to have the meeting, I said the client "you'll be so busy that day, so we should stop it". 

Truly I didn't want to have the meeting because I didn't decide to make the story of the meeting. I expect this proposal pass smoothly, but the PM said "Don't worry. We can manage the time. So the meeting hold normaly". I understood I failed to adjust.

I was too optimistic. I studied the we can't expect the reaction of the others. Our forecast is generally not ture. We have to communicate carefully.