This wil be little complicted. You have to use controltoolbar combo box.
Take a print out and try. If you do two or three time you become faimialr.
1.Open a sheet
2.Go to any column for e.g. G1
3. Type Sunday Monday Tuesday etc up to Saturday in G1 to G7(if it is available as custom list. You have to type Sunday in G1 and copy it down up to G7)
4. click view(menubar)-toolbars-controlboxtoolbar
5. you will get toobox. On the 5th row on the iight is the combo boc
6. click that box and +sign will come and draw a rectangle anyawhere in the sheet.
7. right click the combobox and click properties
8.inthe properties window against "linked cell(the list is alphabetic) type A1 the cell where you need the validation
9.listfll range type G1:G7
10. listrows as 7 and linked cell as $A$1
11.matchentry if you click the arrow you get three choices. Choose choice no. 1 "fm match entry complete"
12. close the properties window
13. click view-toobars-exitdesigbmode
14. in the combobox type s you will see Sunday and also same entry in A1
15.delete entry in A1 the entry in combo box also disappears
16. now type T you will get Tuesday. If you want Thursday type Th. You will see same entry in A1