Something run-of-the-mill is average and unexceptional. Nothing unusual happens during a run-of-the-mill day.
Originally, this term has to do with material produced by a mill — such as lumber — that had the same size and quality. From there, it came to refer to other things that don't vary. A run-of-the-mill baseball player is ordinary: not great, but not terrible. A run-of-the-mill school grade is C. If something is very good or bad, it's noteworthy and therefore not run-of-the-mill.