I fish for them year round. They're too much fun to catch to just focus on the spawning times. Because of my work schedule, I don't get to start serious bluegilling until late April, though.
Early and late in the day, a dry fly or popper is fun. At other times, a maggot under a float is my bread-and-butter technique. If they're not spawning, I look for them in the 8-15 foot depths near their spawning grounds.