Hi meggen

You want conditional formating to change the cells' values in case it's a monday? - That's not possible, as it says it can apply a cond. format, but it can't change any value!

To underline saturdays and sundays mark the whole range you want to apply the formating,
---> choose from the ribbon "Conditonal Formating"
---> new rule
In the dialog choose
---> based on formula
and enter the formula: =weekday($A2,3)>4
Klick on format and choose the format you like to be applied.