We visited in January 2017 and I'd love to go back when there's more than 5 hours of daylight. The trip was spectacular. And it's an easy flight from DC and 5 nights was plenty to hit most of the easy tourist spots.
Winter is the ideal time to go northern lights hunting. Either way, I've been propably too often, 5 times by now, twice in summer including one highland crossing, and three times in winter. My recomondation would be, if budget and vacation time allows, to visit Iceland once in summer and once in winter.
