If you're one of those people who starts getting excited for all things autumn and Halloween as soon as Labor Day is over, this should get you really excited: This year's harvest moon occurs Friday, which just so happens to be Friday the 13th. The harvest moon is the full moon that occurs closest to the autumnal equinox each year, the Old Farmer's Almanac explains; this year's equinox, which marks the first day of autumn, falls on September 23. As CNN explains, during a harvest moon, the moon is positioned at its "most shallow angle" with the eastern horizon, meaning there is less time between the sunset and the moon's rise—typically just 25 minutes in most of the northern US, half the time that usually elapses between the two. That occurs for several nights, giving farmers bright moonlight to work in the early evening right around the fall harvesting time.
As for why the harvest moon often looks reddish-orange, that's because the closer to the horizon the moon gets, the more red it appears, thanks to the way light photons travel through atmosphere. Not only is the timing of this year's harvest moon rare—CNET reports a nationwide full moon hasn't fallen on a Friday the 13th since 2000, and won't again until 2049—but it's a "micro moon." The moon will be near its apogee, the point in its orbit at which it is the farthest away from Earth. So it will look 14% smaller than a typical full moon; per NBC News, it will also appear slightly dimmer. The Farmers' Almanac reports that the moon will actually turn full just after midnight Saturday in the Eastern time zone; for the rest of the country, that moment will occur before midnight. But an astrophysicist says you'll get your best view about half an hour after sunset. (Read more harvest moon stories.)