Change Value of Subsequent Cells Once A Condition Is Met

The simplified story:

I have a user input "Start Month" that is between 1-12.

I have Columns A:M labeled accordingly 1-12.

The question:

If the user inputs "1", I want the cell below column 1 to return "A" and below column 2 to return "B", etc.

However,

If the user inputs "2", I want the cell below column 2 to return "A" and then below column 3 to return "B", and so on and so forth...

The caveat:

Really the return value is a formula based on other cells and not the ABC's, but this was the easiest way for me to explain it.