Saturday is the 21st of December, aka the winter solstice, aka the shortest day of 2019 for everyone living in the Northern Hemisphere. If pagan rituals are your thing, this is probably a big moment for you. If not, the official first day of winter is neat for other reasons, too.

For instance, did you know that a solstice is technically the moment when the sun is directly over the Tropic of Capricorn or 23.5° south latitude? In 2019, this will occur at 11:19 pm Eastern time on Saturday.

The people over at Vox have created a pretty thorough guide to the solstice and the longest night of the year. It tells you why we even have solstices, which, if you didn’t know, is because the Earth spins on a tilted axis. The tilt — possibly caused by a massive object hitting Earth billions of years ago — means that for half the year, the North Pole is pointed toward the sun (as in the picture below). For the other half of the year, the South Pole gets more light. It’s why we have seasons.

Another highlight from the guide is an answer to the question: Is the solstice really the first day of winter? The answer is that it depends on who you’re asking. If it’s a meteorologist, they would say that winter is defined as the coldest three months of the year. Astronomically speaking, yes, winter begins on the winter solstice. For everything you might want to know about Saturday’s winter solstice, check out this guide from Vox.