He enlisted 1940, and spent two years on USS SHAD (SS-235), earned 3 battle stars, then the weird 1 month assignment to Midway Island. His next assignment was USS ODAX (SS-484), which was one of two new submarines used to test out the technologies found on captured German subs. I assume he must have been 'trusted' more than the average sailor by that point?. His first combat tour was surveillance off the coast of North Africa.
After 20 years in the navy , from1962 to 1972 he was CIA based in Vietnam.
Was it R & R, or was it something else?
After the Battle of Midway in 1942, the US Navy expanded the facilities on the Atoll, including the creation of a forward submarine base. By the end of 1943, this rough base had become a major facility for subs heading out on patrol towards Japan... they would stop there on their way out of Pearl Harbor to top off supplies... or coming back from said patrols. Rough repairs could be accomplished there as well before the submarines would go to Pearl for more in-depth work.
If your father-in-law was a submariner, the odds seem pretty good that he was stationed at the submarine base at Midway for that time span. Exactly what he was doing is, of course, an open question that can't really be answered without a lot more information.