When is the best time to split a hive-- early in the season, or later, when the hive is stronger, and the bees exhibit swarming behavior? (I usually make my first inspection and reverse supers in late April-if it ever stops snowing-and swarms occur around the fourth of July.)
I think it's more a matter of strength than it is what month it is. Personally, if I've got two deep boxes full of brood and honey and really full of bees I think it's strong enough to do a split. If this happens as early as May great. As late as July, it's still ok. Later than that kind of depends. I wouldn't go as early as April unless you are buying a queen. If you are letting them raise their own queen there will not be as many drones for mating in April. Even when there are some drones, they need to be mature enough to mate.
You can do splits with less bees, but there is a certain amount of "critical mass" to a bee population. They have to struggle more when the population is lower.
Of course this is all what I would do in Eastern Nebraska. If I lived further North or South I'd have to adjust.
As usual the answer to anything always seems to be "it depends".