You’ve probably started to feel a chill in the air and a crispness in the wind. Leaves are beginning to fall, and you can sleep comfortably at night without turning the fan on full blast. But when does autumn officially begin?

The fall equinox, which signals the official beginning of autumn, is on Monday, September 22 at exactly 10:29 p.m. This means the Earth will experience the same amount of darkness and light. The Earth’s axis is exactly tilted at this time to produce exactly 12 hours of daylight and 12 hours of nighttime.

Due to irregularities in the Earth’s orbit and the Gregorian calendar, the equinox can sometimes fall on September 23 or 24. It’s early this year, but that is not significant for a harder or colder winter.

Additional features occur on the fall equinox that budding scientists and astronomers can note. The sun rises at exactly due east, with the sun setting at exactly due west. The fastest transitions between day and light also occur around the time of the equinox. Additionally, due to seasonal temperature lag, the fall equinox is typically much warmer than the spring equinox, which occurs in March.

Around the fall equinox, the middle latitudes of the Northern Hemisphere see the most significant loss of daylight. Cities such as Washington D.C., New York City, and Boston begin to see two minutes less of daylight per day. In higher latitudes such as Canada and Alaska, the loss is even higher.

The fall equinox marks shorter days and longer nights, followed by colder temperatures. For patients with Seasonal Affective Disorder, a mental condition marked by seasonal depression, it can be a troublesome sign that winter is approaching.

Get your winter clothes out of storage and start stocking up on rock salt, hot cocoa, and baking supplies. Summer is officially over, and it’s time for fall to start rolling in.