How do we decide who gets Friday or Monday (ie, a three day weekend)?

I have a friend who once had M-W off, and worked the four remaining days. He hated it because his time off was completely misaligned with his peers, so he had no social life.

Edit - I don't have any fundamental problem with working fewer hours or fewer days. But, I think I'd prefer 5x7 hour days instead of 4x8.