There are four Seasons in Stardew Valley: Spring, Summer, Fall, and Winter. Each Season lasts 28 days. The game begins on the first day of Spring. After completing a cycle of seasons, the game moves forward one year when Spring starts again. Years are tied to a few events, but there's no limit on the number of years which can be played. A villager's daily routines may change with a new season or with some events.
Spring, Summer, and Fall have two festival days, and Winter has three, (Not including the Night Market Events.) At festivals, the Pelican Town community gathers for special activities. Special items can be bought, games played, prizes won, and information gained.
Most crops, Foraging items, and fish are available only during their particular season or seasons. The seed selection available in Pierre's General Store (or Joja Mart) changes each season. When a season ends (end of day 28), crops wither and die unless they can also grow during the coming season (pay attention to those growing days). At that time, fertilizer used on soil also expires, unless it is on a tile where a continuing multi-season crop is planted.
During seasonal shifts, your farm will also experience minor disrepair: the fallowing of tilled land, and the appearance of random debris that also must be cleared before planting can begin again. Land that is still planted when the season changes does not undergo disrepair.
Some areas are unaffected by the seasons, mainly underground areas as well as The Desert.