I have been down several times the week of Memorial Day. The holiday weekend will be busy, 8-9-10 busy, which is "normal" for a summer month. Because many schools are not out yet, Tuesday on will be a little nicer. Think 6-7. It won't be as busy as mid-June/July (9-10). (although crowds right now are below summer norms). There is also speculation that next summer will be below norms due to people holding off on trips until Star Wars (Galaxy's Edge) opens at the end of the year. I'm not sure if this will impact the regular school age summer trips.
Weather wise will be HOT... By the end of May/early June is the heat of the summer there. Mid to upper 90s each day. Expect afternoon rain each day.
We used to go this week when DS was in school. Schools here are out in mid-May. If you are tied to school calendars, this is probably the best option of the summer.
j