If you are ordering queens, then almost any time will work, although I'd shoot for mid April, or a bit later to get the most number of bees. That is also a good time to be opening up the brood nest. Swarm season starts around the middle of May.
Generally I've read that people can start doing splits when raising their own queens about the time dandelions bloom which is also when the drones start flying.
If you buy a queen the weather is not as much of a factor, since she don't need to mate. So then you just need to wait for a maximum of brood and bees, and there should be plenty 4 weeks from now.