watermark writes "About every 28 years a 'supermoon' occurs. This is when the moon's orbit is closest to earth at the same time as a full moon. Saturday night will be the biggest, brightest full moon you will see in the next 28 years."
The buzzkills at Space.com explain though that (For North Americans at least) you'll actually only be seeing a "waning gibbous moon
," but it should still be spectacular.