>I think your suggestion overcomplicates the situation. I really don't want to make anything more harder than it is. I did disable the option button in addition to making it invisible just in case. The optiongroup is an instance of cOptionGroup class from MereMortals and I can not (and don't want) to make amendements to this class.

To each his/her own. I do think, however, that it's ironic that you say that it (my first suggestion) over complicates things because I think the same about the invisible/disabled option button. In my mind, it's unnecessary and irrelevant to the problem. OTOH, adding a parameter to the DOW() function is simpiler and can be more self-documenting.