Hi Tina,
I think that you'll find that the middle of May is a really pleasant time to visit Walt Disney World. Compared to when you typically visit in July and August, the crowds should be lighter and the weather should be cooler. Because most school districts are in session between May 13 and 18, the crowds should be less than during summer when most children are out of school. I like to visit touringplans.com for predictions of what crowd levels will be in the parks on specific days. In May, you can expect average high temperatures in the upper 80's Fahrenheit, which is still quite warm, but not as hot as in July and August.
During your time at Walt Disney World, you will be able to take advantage of a couple of special happenings.
The Epcot International Flower and Garden Festival runs from March 6 - May 19 this year. At this event, you can see beautiful topiaries around Epcot, try food at the Garden Marketplaces, take in great concerts at the Flower Power Concert Series, and so much more! Also during your time at Walt Disney World,
Star Wars Weekends will be held at Disney's Hollywood Studios beginning on May 17. You can meet celebrities from the Star Wars universe, and take in special shows such as the Hyperspace Hoopla!
I hope that this information helps you in planning a magical vacation, Tina. Please do not hesitate to ask if you have any further questions.