Memorial Day weekend has lately been the kick-off to the summer season in the parks. So, I would anticipate good sized crowds on the actual holiday weekend (Fri-Mon), especially if they open or debut something new that weekend. The weekdays (Tue to Thu) would probably be okay (relatively speaking). Just note that there aren't that many days when the parks are "empty" anymore (i.e., attractions are walk-on or near walk-on), so it's best to go expecting at least a moderate level of crowds, even on weekdays.
(Side note: There are a number of Grad Nights slated for that week - May 29 & 30, Jun 1 & 2 - which might increase crowds depending on when the various school groups begin to arrive and which ticket tier they purchased.)