One-Letter Abbreviation for Weekday

Without creating a separate column, how can I format the weekday portion of a date like this (N, M, T, W, R, F, S, for Sunday through Saturday)? For example: R.1/1/09. I'd like to use the custom number format function.

=choose(weekday(a1),"n","m","t","w","r","f","s")

Thanks for the quick response. I was trying to avoid having to add an extra column for the "choose" command. I know excel number formatting "ddd" gives "Sun, Mon,…" and "mmmmm" gives "J, F, M, A, M, J, J,…" for the months. I was hoping Excel had some undocumented code (but known by some, of course) where I could create my own cell number format maybe by creating my own [$-####] code.