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.