The plural of ‘Monday’ is ‘Mondays.’
In English, to form the plural of most nouns, you typically add an ‘s’ at the end. Since ‘Monday’ is a common noun referring to the second day of the week, following this rule makes it simple. So when you refer to more than one Monday, you just say ‘Mondays.’ For example, you might say, ‘I have meetings on Mondays.’