You can use data validation to restrict the type of data or values that users enter into cells.
You can choose from three types of error alerts: Warn users that the data they entered is invalid, without preventing them from entering it.
For the Error Alert I checked Show error alert .... For some reason that will allow the selection of Y and N and any other text triggers the error message. Joe If the user enters an invalid value, they'll see an error message.
However, if I have the cell selected and hit the Delete button it will let me delete the cell without a warning. If ' Ignore blanks' is unchecked, and they click Retry, then press the Delete key, they'll see another error message, because the blank cell won't be allowed as a replacement.
If only a short range of dates is valid, such as yesterday, today and tomorrow, you can create a drop down list of those dates.
By using formulas, the drop down list will be dynamic, and will show the correct dates whenever the workbook is opened.