Same Time, Next Year (1978)

"They couldn't have celebrated happier anniversaries if they were married to each other."

A man and woman meet by chance at a romantic inn over dinner and, although both are happily married to others, they find themselves in the same bed the next morning questioning how this could have happened. They agree to meet on the same weekend each year—in the same hotel room—and the years pass each has some personal crisis that the other helps them through, often without both of them understanding what is going on.