How long is a year? What makes a season? What's Equinox? What's “Tropic of Cancer” and “Tropic of Capricorn” you see on maps such as Google Earth.
Why is there so many different calendars from so many old civilizations? For example, you hear about Chinese year, Gregorian calendar, Hebrew calendar, islamic calendar. They all disagree with each other, seems annoying. Can't we just have days starting from 1 and define a year by 100 days? That would make life much simpler.
These are age old questions. The seasons, is the most important thing that started science and mathematics, since ancient civilizations. Why is it important? Because, successful crop raising depends on knowing the seasons well. Without which, you have hunger.
And, to know the seasons, means knowing the pattern of when the weather will be cold, hot, snow, rain. Because, the plants go by that. And to know the seasons, our ancestors observed that it depends on the paths of the sun, the moon, the stars. Once you know the seasons well, then you can device calendars, which is essentially a easy way to mark the seasons by day. There are so many different calendars because, as we now know, that the seasons came from earth's orbit, and it is very, very, complex.
Here's a fantastic video that introduces the subject.
ain't it a wonder? Now, that's why you study geometry. More specifically, analytic geometry, usually part of your calculus.
Here's some Wikipedia links for further reading.
In modern day, when we think of calendars, we think of weekends, holidays, appointments. We forgot, that the primary purpose of calendar isn't about keeping dates, but tracking seasons. Because in modern society, most of us never have thought about crops and its connection to seasons. When you are hungry, you just order a pizza or lamb chop.