1. Select the Cell > Data Tab > Data Validation > Data Validation
2. Under Settings Tab, in Allow, select Custom
3. Let's assume that we want users to enter only those dates which are Wednesdays. Put following formula in
=WEEKDAY(A1)=4
4 is for Wednesdayday. 1 is for Sunday and 7 is for Saturday other values lying in between.
4. Customize your Input and Error Message as per your requirements.