Unexceptional describes something that's perfectly ordinary, like your unexceptional day that's not good or bad — or memorable for any reason.
Anything that's an everyday, normal occurrence can be called unexceptional. Brushing your teeth, a regular day at school, and a so-so movie: they're all unexceptional. Think about the word exception — something that stands out or breaks a rule. So something that's exceptional is special in some way. Adding the prefix un- makes an adjective that describes something that's not special, like an unexceptional bus ride to school.