Ted -

Here is a list flies that you might expect in May:

Hendricksons

Paraleps/Blue Quills

Apple Caddis

Sulphur

Cornuta

Green Drake

Blue Wing Olive

Black Caddis

Grey Fox

Unless you're obligated to spend the entire 2 days on the Beaverkill, you might also wish to drive over to the West Branch of the Delaware and give it a shot as well. However, 2 days is hardly enough time to more than get your feet wet. The end of May should put you right in the midst of the year's best fishing, and most prolific hatches, but be prepared to share the water with lots of other fishermen.