The old 'shiny bike in the rain' conundrum! If you have another bike to use on wet days then cancel and rebook when it looks like decent weather, if not then your BM is going to get mucky sometime so may as well be for it's first MOT. If you're going to be using it over over the weekend then a quick wash (hose not jet wash my preference but each to their own), wipe dry best you can and then give it a good clean when you next lay it up for a few days.