an option is to make a dropdown menu where people can select a date.
create a second sheet with dates en use data validation (list) that refers to the second sheet.

you have to enter the reference to the second sheet yourself because excel'07 "can't" do it.
enter for example "=Sheet2!$b$2:$B$100" in the source

i think you have to go in VBA mode to actually make an calandar like pop up.

Best regards,

Ysball