Working as a zookeeper requires you to be passionate about animals and wildlife conservation. You often spend more time with the animals than other zoo employees, getting to know their habits and personalities. This helps you determine when the animals are healthy or whether they need veterinary care, a vital role of primary caregivers.
So on average the number of days between two similar phases (e.g. between one full moon and the next full moon) is about 29.531 days. This period is referred to as a lunation, lunar month, or synodic month.
So there's time for (365.23/29.53) = 12.37 periods between full moons, or an absolute max of 13.37 full moons in one year. With only 12 months in the year, you can't squeeze two full moons into very many of them. And that's why it doesn't happen too often.