The 2020 winter solstice occurs on Monday, December 21st, at 2:02am pacific time. It happens, of course, at the exact same moment around the world, and you can see what time it will occur in your neck of the woods here. The solstice and the seasons occur because the Earth is tilted 23.5 degrees away from the sun, and as it becomes cooler in the north, the Earth tilts gradually away from the sun. This means that the southern hemisphere becomes warmer and experiences summer when in the north hemisphere experiences winter. The solstice happens when the tilt is at its most extreme angle. (For an easy-to-follow explanation of the seasons, click here.)
The solstice is a rather unimpressive event, as opposed to a lunar or solar eclipse. You probably won’t see any dramatic shifts in the skies, but at noon, shadows made by the sun will be the longest of the year, because the sun’s arc in the sky has been dropping lower and becoming shorter since June. You may also notice that the sun seems to be rising and setting in the same place for the same reason. This is actually how the word “solstice” came to be – it means “sun stands still” in Latin. National Geographic has a great explanation of how a solstice occurs here.