Two full moons in a month is called a blue moon. However it happens a lot more often than every 2000 years. The average time between full moons is 29.53 days. The average month length is 365.25/12=30.44 days. So the full moon moves on average 0.91 days earlier each month. That means it happens twice in a month on average every 30/0.91 month or 33 months, or every 2.7 years or so.