If you do not like "lustro" you can say "zwierciadło" but it's old-fashioned word.

It appears it is old Latin word lustrum that denoted ceremony performed every five years.

In Polish one (if he had to) would use pięciolatka. It came from Soviet 5-year economical programs, because it normally means 5 years old girl.

I forgot the most obvious version: pięciolecie.