Krishna Paksha is the period of fifteen days. It starts on the Full Moon day (Purnima) and peaks on the New Moon day (Amavasya). Generally, there are 12 Full Moons in the English calendar. But, if a situation of Blue Moon occurs, there would be 2 Full Moons in a single month, ending as 13 Purnimas in a year. Amavasya is the moonless night. Just like Purnima, there are 12 New Moon days in a single year.