Checkbox error message

Hi,

I'm playing around with checkboxes for the first time and dipping my toes into VBA and really could do with a little help from those with a far better understanding than me.

I've got a check box in one cell (checkbox1) and I also have another cell (we'll say "D10") and what I am trying to do is link an error message box to appear when the checkbox is selected and if the cell contains either a 1 or 7 (Sunday, Saturday).

The idea being that if someone clicks on the check box and the Weekday in cell "D10" is either a Saturday or Sunday a message box appears and warns them that they've selected a weekend day (which they can do in certain circumstances but need the error message to warn them of the fact)

Any help gratefully received.