A legit excuse to tell your spouse you would rather hang out with your male friends on that day than engage in trivial activities with the aforementioned.
Spouse: We're having dinner with my coworker Janet and her boring husband Ben tomorrow.
You: Can't do darling, Saturdays are for the boys.
Saturday's happen to be for the boys. Saturday's are a day when instead of staying with your spouse or significant other, male specimens spend time with their male friends. Saturday's are for the boys.
Saturdays are typically when college football games are played, while NFL games are mostly played on Sundays. College football players = boys, NFL players = men