Used as a noun or adverb to mean 'each night' or 'nightly'. Common in everyday speech and writing.
毎晩、寝る前に本を読む。
Every night, I read a book before going to sleep.
彼は毎晩遅くまで働いている。
He works late every night.
My mother writes in her diary every night.
He works overtime almost every night.