I've heard it so many times I have no idea where it originated but common wisdom is that:
"A swarm in May is worth a load of hay.
A swarm in June is worth a silver spoon.
A swarm in July isn't worth a fly."
My addition:
"A swarm in April will probably catch a beekeeper by surprise."
This is the earliest swarm I've ever seen, April 20th. About 2 lbs of bees and we found them at 4pm in the tree. With this I am officially out of bottom boards and covers. I also have more hives than I want to have with 5 hives that need splitting (or something!) or they will likely be up in the trees too.
