Busy or not, how hard is it to post a "hey, we've got some last minute changes to make and the calendar will be out late as a result" or whatever the reason may be.
I don't even particularly care about the calendar, but this seems like a really stupid thing to not have any communication on. Complete radio silence is almost never an acceptable way to handle something that your clientele is expecting (and has reason to expect) regardless of the business you're in. "Tell them before they have to ask" is a pretty easy thing to do in this case, and almost always results in a much less negative response.