Given a conditional statement
[tex]p\implies q[/tex]
the converse statement is obtained by swapping hypothesis and conclusion:
[tex]q\implies p[/tex]
So, the converse statement of "if tomorrow is monday, then today is a weekday." is "if today is a weekday, then tomorrow is monday."
This statement is false: the counterexample is if today is Saturday: it's a weekday, but tomorrow is not Monday.