The moon comes back to the same place every 29.5 days. So in every month except February, there must be at least one new moon. In February, if there is a new moon on the last day of January, there will not be another new moon until the first of March. Presumably this happens about 3% of the time (given there are 31 days in January.) So on average every 3-ish years there is a February without a new moon. It's a little more complicated than that because of leap years, but it's about right.