Seasons! Minecraft now has a yearly cycle of seasons. The game starts in Spring (approximately May 1st) and a year lasts (by default) 128 days. The config file contains suggested settings for both SMP and SSP, by modifying how long the moon takes to change phase (number of nights per each of the 8 frames). One full cycle of the moon is a "month" with 2 "months" per season and 4 seasons (below) per year.
Thus, by default of 2 days per phase, the year is 2*8*2*4 = 128 days.
- Spring is standard temp, plus or minus, and very wet.
- Summer is very warm, and dry.
- Fall is colder than normal, and very dry.
- Winter is very cold and slightly wet.
Be aware that these are modifiers on top of the base. Cold Taiga is never going to get above freezing, even in the summer, and the desert isn't going to get below freezing. Additionally some biomes fluctuate less (the savana sees half as much temperature and 2/3rds as much rain) and others are offset slightly so that the fluctuations don't cause oddities (deserts are offset downward on rainfall, swamps are offset upward on temperature).
Vanilla causes water to freeze at around 15°F (0.15°MC, -9°C), which is cold even for an oceanic freezing temperature, but close enough to not bother with additional ASM modifications. Harder Wildlife causes ice to melt back into water at 20°F (0.2°MC, -6°C), although the rate varies, increasing as the temperature gets higher with some impact from sunlight. Due to blocks updating at random, this may cause some ice to persist.
Weather (downfall) changes as is appropriate for the biome conditions: that is, it snows when the temperature is below 0.2, it rains above that (and has positive rainfall), and stops raining if the temp reaches 1.5. Most of this is handled by vanilla, so few changes were needed other than tweaking the biome parameters. Crop growth is also affected by the changes in seasons, generally slowing down in the fall and winter and picking up during spring and summer.
Below is the generalized graph showing the temperature and rainfall offsets over the year (ignoring the specific modifiers stated above). Month names are used for convenience, instead of world tick times. Specific values have changed since this graph was made, as well as some biomes flatening or enhancing the curve, so this should be treated as a broad overview.
In-game, the current weather can be determined by using a Thermometer or a Hydrometer, including determining how appropriate the weather is for specific crops (see the associated item pages for details). Additionally this information can be extracted as a redstone signal using an Item Frame and a comparator.