The first day of the week, following Sunday and preceding Tuesday. In Japanese culture, it's associated with the moon, as '月' means moon. It's often the start of the work or school week.

一(いち)週(しゅう)間(かん)の始(はじ)まりの曜(よう)日(び)で、日(にち)曜(よう)日(び)の次(つぎ)に来(く)ます。「月(つき)」は月(つき)を表(あらわ)します。