Days are a unit of time first introduced in Creatures 2. In that game, each day is thirty minutes long, and days are divided into five parts - dawn, morning, afternoon, evening, and night - which each last six minutes. Four days make up a season, and therefore seasons are two hours long. Some things only happen on a particular day. For example, Zander fish eggs only hatch on the third day of autumn.
In Creatures 3 and DS, seasons are four days long by default, and each day is 20 minutes long (making one season 80 minutes long). Each day is further evenly subdivided into five different times as in Creatures 2.
None of the agents that come with the game are affected by the different days or times of day, but some third-party agents take advantage of this system. Several can show the day or time of day, such as the Calendar Wheel; others change behavior based on time of day, such as the Sunflower or the Moonflowers in the Desert Ruins, or occasionally based on the day of the season.
The GAME variable engine_LengthOfSeasonInDays controls the length of a season in days, and the variable engine_LengthOfDayInMinutes controls the length of a day in minutes.