In the Roman Empire there had been one day each year where masters and slaves did change roles for that day. Didn`t work out well, as all the other days the masters would have the chance of revenge.
But, within a monogamous relationship, i love the thought of an
anti-anniversary. It would be half a year later following each anniversary with the rule that both partners are free to find and fuck anyone they like, but only on that special day.
That could lead to a competition in which each partner tries to be the first to find a playmate or is aimed at a greater number of mates at the end of the day.
The following day they would tell each other about the experiences they`ve made the day before. - Would spice up and revive a couple`s sex life for sure!
i think this idea should be brought up to couples attending a relationship/sex counselor.