Go to the Properties | General tab and enter an expression into the conditional calculation box. Something like:

GetSelectedCount(Year) > 0 And GetSelectedCount(Month) > 0

This will cause a Calculation condition unfulfilled error to appear if there is no Year or no Month (or neither) selected.  Change this error message to a more relevant one by clicking Error Messages, select Calculation condition unfulfilled and type your desired error messge.

If Year and Month are not the correct field names, change the expression above to the correct field names.