I love going to Disney World during the Flower and Garden Festival.  It is absolutely gorgeous at Epcot.  The first time I attended was during our honeymoon in 2013.  I actually did not really know what it was, but when we arrived, it was stunning.  We went back for our 2nd anniversary as well. They also used to have Star Wars Weekends, but they no longer do that anymore.  We have found May to be great because it isn't very crowded, but the weather can be fairly warm.  I have written two previous posts regarding what to wear to Disney World in January and in September.  I find May to be very similar to September.  It is actually a little bit cooler in May, but not by much.  The highs are usually in the mid-to-upper 80's, and the lows are usually in the high 60's.  Generally I recommend shorts/skirts, short sleeve shirts, or dresses.  Below are some of the specific outfits both Brad and I have worn.  I hope this helps if you are planning a trip around May!