...because this is the peak night of the annual Perseids meteor shower.
From NASA's website:
"Earth passes through the densest part of the debris stream sometime on August 12th. Then, you could see dozens of meteors per hour."
For sky watchers in North America, the watch begins after nightfall on August 11th and continues until sunrise on the 12th. Veteran observers suggest the following strategy: Unfold a blanket on a flat patch of ground. (Note: The middle of your street is not a good choice.) Lie down and look up. Perseids can appear in any part of the sky, their tails all pointing back to the shower's radiant in the constellation Perseus. Get away from city lights if you can.
Above: Looking northeast around midnight on August 11th-12th. The red dot is the Perseid radiant. Although Perseid meteors can appear in any part of the sky, all of their tails will point back to the radiant. Image copyright: Spaceweather.com, used with permission.
There is one light you cannot escape on August 12th. The 55% gibbous Moon will glare down from the constellation Aries just next door to the shower's radiant in Perseus. The Moon is beautiful, but don't stare at it. Bright moonlight ruins night vision and it will wipe out any faint Perseids in that part of the sky."
They advise that the time of evening that best reduces glare and increases the chance of seeing "earthgrazers", those with long, extra bright streaks, is between 9 and 11 PM in your local time zone, and there will be a "double peak" for quantity, between 11PM and 1AM, and again around 5AM tomorrow. Yes, I know it's prime posting time here, but everybody will be here tomorrow night; the meteors won't!
Last year, it rained or was cloudy here in the Northeast, but tonight the weather is looking very promising, so get outside, enjoy the show, and don't forget the bug spray!