I also like the second week of December, because the resort will generally be in full holiday mode at that point. It means long(ish) hours, full entertainment, and lots of decorations. That being said, there are a few caveats worth mentioning:
- The weekdays are generally good, but the weekend will probably be fairly busy because it's the last weekend/partial weekend available to Deluxe, SoCal, and SoCal Select AP holders before the holiday season blockout goes into effect. The holiday blockout starts on Friday, December 18, 2015. So you'll see a fairly pronounced uptick in crowds from Fri nights through Sunday (compared to Mon-Thu).
- Some of the holiday merchandise will start to be running thin. There will still be a lot of stuff, but they may already be sold out of some items (e.g., pins) and other things might be running low (e.g., specific sizes of clothing).
In terms of the Spring, that's a little trickier because Spring Break doesn't happen at the same time every year. It usually happens sometime in March and April (Easter is on April 5 in 2015 and March 27 in 2016). Spring Break generally means longer operating hours and full entertainment, but it can also mean high(ish) crowds. While this is not a hard and fast rule, I would anticipate "spring break" to cover the three weeks before Easter and the one week afterwards (it doesn't not always follow this pattern, but it's a starting point).
February, except for Presidents Day weekend, tends to be on the quieter side. As mentioned, there are generally a fair number of refurbishments, the parks are operating on shorter hours, and there is less entertainment (fireworks and F! are generally only offered on weekends and holidays, but the parades and WoC are nightly). Now, how things will work out in 2016 is less clear because of the 60th anniversary starting this year.
