Rainy days can be one of the best times to hunt. One, it will soften the leaves so you can move through the timber quieter. Two, it will sometimes create a sense of security for the deer. Maybe they think nothing else would be out moving in the rain(light rain/drizzle). Three, being set in your spot, for when the rains stops or lets up. Deer will almost always stay holed up during a heavy rain. When the rain stops, they will move out of cover to feed, and travel. Of course, sitting out in a thunder/lightning storm is not the best of ideas though.