Not sure if they have more special stuff for Memorial Day, but generally in the summer they have Star Spangled Tuesdays. Lots of pool games and activities that evening, plus the fireworks are on Tuesday nights. They may make some adjustements for the holiday though.
Every day at HH has tons of activities. We were so busy there this past August we hardly had time to relax, thus we had to buy an add-on there just to take enough vacations to appreciate it! My kid's inheritance is being drained by the Mouse and the fantastic DVC vacations we have fallen in love with!!