If you really have a table with six period fields, the first advise I have is to go back to table design and get that right first, then worry about other stuff.
You want your user to choose the field so they can select a specific period? Can't be done with a query because your table design is totally wrong. You'll have to code it. Unless of course you choose to listen to my first peice of advise... scrap what you have and design it correctly and you can then do it with a normal query.