It's happening tomorrow (Friday, July 31): the second of two full moons in the same month.
The moon won't actually look blue, but it's certainly a rare occasion, which is why we use that expression, "once in a blue moon."
Here's a San Francisco news station's description of the upcoming event.
Good news if you enjoy looking at the full moon; not so great if you want to look at the Delta Aquarid meteor shower.
The next blue moon will occur in January of 2018.