The day of the week Monday (the second day of the week). Often shortened to "Mon." in everyday use.
月曜日はいつも会議がある。
I always have meetings on Mondays.
毎週月曜日に日本語の授業があります。
There is a Japanese class every Monday.
今週の月曜日は祝日だったので会社は休みでした。
This Monday was a holiday, so the company was closed.