Eight seasons: Winter, Winter-Spring, Spring, Spring-Summer, Summer, Summer-Autumn, Autumn, Autumn-Winter. A month and a half apart.
It's about where the Sun travels. Winter, for example, doesn't mean what a SL/OS veteran may expect, namely a somewhat wintery mood, but the Sun goes through the zenith and stands right above you at noon as it always does. Rather, it means e.g. you pick the daycycles for 40° northern latitude, and the Sun doesn't rise any higher than it does in real life in late January/early February at 40° northern latitude, and the day is as long as it normally is at 40° northern latitude.
So for a Christmas-themed sim, you might want to pick an autumn-winter daycycle that matches the latitude of wherever in the world your sim is supposed to be located. That will give you a low noon Sun, a short day, a long night, just like in real-life mid-December. Or, depending on the latitude, the Sun may actually not rise at all.
Easter sim: spring. Halloween sim: autumn.
It's eight because that's a compromise between having nothing between the solstices and equinoxes and having to calculate and manually build even more daycycles (e.g. twelve if there shall be one for each month).
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