A decent hatch chart. But the sulphurs start much earlier than May 20.
The size 14s hatch first, then the size 16s, then the size 18s.
The size 14s start hatching much earlier than May 20. It's not unusual to see some even in late April (including this year). I've seen heavy hatches as early as May 4. They are on already on many streams, so have some in your box if you're fishing this weekend.