I normally raise some queens to make splits with. In order to get queens mated, you want to see drone pupae at the purple eye stage before you start raising queens so the drones are mature enough for mating when the queens are ready for their mating flights. Here in Ohio, I usually start seeing drones at the purple eye stage the first week of April.
In my experience, the week around May 10 seems to be when a lot of swarms happen. Swarm prep happens for the hive 3 weeks before a swarm happens. (That means I need to be making splits 3 weeks before the main swarming season.)
Once I see purple eye drones, I start grafting, and 10 days later I am making splits. I generally feel like I have a one week window to work with as soon as I start seeing purple eye stage drones. If I wait to start grafting more than a week after purple eye stage drones, by the time I do splits the hives are already making swarm preparations. (Don't get me wrong, sometimes splitting a hive will knock the swarm impulse out of them.) Do I always get everything done in the one week window? No, but that is the goal and I try to get the bulk of the work done.
Another method that may be easier for hobbyists is to remove the queen and a few frames of bees and brood once you see purple eye drones. Make a nuc with the queen and bees. The old hive will raise a new queen. Removing the queen will prevent the original hive from swarming unless they are already making queen cells and preparing to swarm.
If you want to buy mated queens, you can make splits whenever you want, but new queens will be accepted a lot better if you have a little nectar flow coming in.
The weather won't be the same every year, so the first week of April for drones and May 10 for swarms are guidelines and not written in stone. Sometimes it happens a couple weeks ahead of schedule, and sometimes you are a couple weeks late.
My concern is swarm prevention for next year, just want to be prepped.
Plenty of drawn comb above the cluster long before you think the bees will need it. I have seen bees make 20 or 30 pounds of honey from maple trees from a week of warm weather in February before. Keep an eye on your hives, and if they have nectar coming in, make sure they have room to store it.
Allen gave a neat trick in his diary years ago. In order to help the bees regulate the temperature in the hive, but still give them extra space, place a sheet of newspaper between each super. When the bees need the space, they will chew through the paper, but if they don't need it, the paper will help hold the heat down with the cluster.