In Excel, a single cell (or even an entire column) can be given a data validation setting so that, when selected, a dropdown list appears with a finite list the possible entries for that range.
Drop-Down list example allows a user to select a day of the week
Steps:
Select the cell or range of cells that is to contain the dropdown list box.
This may be either a single cell or a larger range
Click Data, then Validation.
Click the Settings tab (if necessary)
Under Allow, click the down arrow and select List.
Using the Source box either enter the range (A-direct list) or refer to a range (B-referenced list).
A - Type your list into the Source box. Separate items with commas with no spaces in between.
B - From within the Source Box either click and drag to select a range or type = and enter the range coordinates.
Notes
If a user tries to manually enter anything other than these values, a stop message appears and will not allow the cell to keep the invalid entry. The only options for the user will be to Retry or Cancel.
It will be possible to make valid entries (i.e., MON, TUE, WED, etc.) manually in a cell to which this validation setting has been applied; a user is not required to actually select these values from the list.
Drop-down lists (or manual entry) is not case sensitive; Wed, WED, wed, etc are all treated the same.