It also depends on whether you want to see DLR with or without Christmas/Holiday decor. If you go in early December 2015, most likely the parks will be fully decorated for the Holidays. Assuming 2015 is similar to 2014, this means the Haunted Mansion will be running the Nightmare Before Christmas overlay and It's a Small World will have its holiday overlay in place. There should also be a lot of decor throughout the parks and hotels. It's already been announced that the usual Christmas fireworks show will
not be presented because of the new 60th fireworks show (
Disneyland Forever), but there hasn't been any concrete information about WoC Winter Dreams or the Christmas Parade. If you do end up going in December, I recommend checking out the
Christmas/Holiday SuperThread.
December tends to be a pretty busy month. Depending on when you go, I would expect moderate crowds on the weekdays (Mon-Thu) and high crowds on the weekends (Fri to Sun). The weekends are popular because a lot of AP holders want to see the holiday decor and offerings, but have to go before the seasonal blockouts start (Dec 18 or 19). There is also the Candlelight Processional (usually the first weekend in December). Once the blockouts go into effect, you start to see a ramp up of non-AP visitors, so there is really no quiet period after the middle of December. The trade off is that very few attractions have planned closures during that period and the parks should be operating on long hours.
In terms of 2016, it looks like Easter is on March 27, 2016. So that means the "Spring Break-Season" is likely to start in early March. If you go before "Spring Break-Season" has started, you're likely to encounter lower crowds, but with more planned closures and shorter park operating hours. Also, there is generally less entertainment offered (e.g., Fantasmic! and fireworks are generally only on weekends (Fri to Sun)). This year has a lot of refurbishments planned during the winter/spring (including "Spring Break-season") in preparation for the 60th, so this is somewhat of an anomalous year. I don't think there will be as many things down next year, but there will still be winter/spring closures (most likely one of the water rides). Haunted Mansion usually re-open in late January (it closes when the holiday season ends to have the overlay remove) and IaSW usually closes in late-January through early-February, so both should be open by late-February/early-March.
If you end up selecting dates in early March, but after "Spring Break-season" has started, then the parks should be operating on longer hours with more evening entertainment. Crowds will also pick-up. West Coast school districts do not all take the same week off for "Spring Break", but it generally occurs in the several weeks before and after Easter. I think that is why "Spring Break" is more like "Spring Break-season" at DLR (it spans about four weeks). If you want to get a sense of this year's schedules, I've been tracking key dates in the
2015 Calendar of Events & School Breaks thread. I don't think crowds will really get to heavy until closer to Easter (the week before and after).