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.