Since the Moon's period of phase is 29 and a half days, while a month usually have 30 or 31 days, it's obvious that if a full Moon lands on the first day of any month except again in February, it will repeat again by the end of the month? Well, it turns out the calendar of blue moons happens every 30 months on average, two and a half years. Seasonal blue moons happen at a similar rate about once every two to three years