Monday's conditions are *not* forecast to be terrible. Light rain, moderate wind, and temperatures in the 50's. We get that in the fall too. It's a road race. You dress for the weather and you deal with it. Man up.
The Boston Marathon is run on Patriots Day, a Massachusetts State holiday and very special day for the city of Boston and neighboring towns. Patriots day celebrates the battles of Lexington and Concord. These happened on April 19, 1775, not some random October weekend. It will never be moved to a different day.