Seasons are a mechanic that was added in version 1.13.0. There are four seasons: Spring, Summer, Autumn, and Winter. Each season lasts 3 month, or 27 in-game days. Full year therefore takes 108 days to pass.
As the seasons change, the color of the foliage shifts (though it does not affect the color of flowers and crops as of 1.16). The month before the season officially turns, the foliage will take on the cast of the upcoming season.
The exact effect of the current season depends on the area of the map that the player is in. The further north you travel, the colder and harsher winters will become, while the south features hotter temperatures.
The first month of Spring is March.
Default worldgen starts in the spring (specifically, May 1st). Trees and grass are vibrant green. Rain is more frequent in this season.
The first month of Summer is June.
Summer features higher temperatures, and paler foliage. Some crops, such as Turnips, will struggle to thrive due to the high temperatures.
The first month of Autumn is September. The foliage of most plants, especially trees, turns bright red and orange.
The first month of Winter is December. The foliage of plants will become pale gray. If the seraph stays in the default starting location, they will regularly experience freezing Temperatures and even snow. As temperatures will regularly drop below -10°C, crops will struggle to survive even in a greenhouse as greenhouses only increase the temperature by 5°C.
Snow pileups can become a major issue in winter, as a depth of 2 is enough to allow mobs to jump over fences, including animals and drifters. Further more, snow of that depth will also prevent grass from being harvested.