You can call St. Michaels Marina and also the Harbor Inn and Marina. They will let you know. Are you planning on staying overnight? If not they may have a spot for you for a few hrs. or so. Most boats will not come in until sometime Fri. You may luck out. Also the water taxi will come out and get you at the anchorage spots.
If you plan on making other trips to St. Michaels marina then I would join the Maritime Museum, they always have room for a boats 20-25'for the day.
As you come into St. Mikes the Crab Claw rest. is on your stbd. just past them is a small marina that also has day slips. You could hail them when approaching and ask them as well. I would call all of them now and see what is available. Dont give up, ask for the dockmaster. They will all try and accomodate you if you are persistant.