Showing up to help move on a rainy day is what “I’ll be there” actually means.

A promise made in good weather has not been tested yet. What it means to say you will be somewhere only gets settled on the morning when everything about the day argues against going: the rain, the early start, the better offer that turned up later. Going anyway is what turns a sentence into something other people can build on.

Go back over what you have already agreed to this month and find the one you are quietly hoping gets cancelled. Decide now that you are going, and say so out loud. If you truly cannot, tell them today rather than on the morning.