It is hard to predict one year to the next, but in 10 years of trips 3-5x/year I can comfortably say that regardless of when you go, it will be more crowded than the precious year at the same time.
Two years ago first Saturday in May-Wednesday wait times for headliners were 30-45 min at 11 am.; this year they were 50-70 minutes.
Schools let out at different times in December and colleges tend to be out the end of the first full week. Our public schools let out the 11th. Still, it is a very good time to go and as long as you do RD, you will be fine! MK and AK are IMHO the most critical parks to do RD, then leave at 1 or 2 for rest, swim, nap, mini golf, etc. When _I_ do MK or AK in the morning, I *always* hop to a different park in the afternoon.