I have 300 cupcakes ready to be filled and frosted - but they are for an event on Saturday morning. When should I fill and frost (with a buttercream) these cupcakes to make sure they are done in time, but without the filling making them soggy?

What type of filling is it? Most of mine don't make them soggy because they aren't that loose. I have filled and frosted a day to two ahead of time. I do it assembly line style. Fill all, then frost all while in their boxes.