Amavasya and Purnima correspond to the two extreme states of the waxing waning cycle of the moon. While amavasya is called no moon day or new moon day, Purnima is called full moon day. Moon being the natural satellite of earth and being seen in sky graciously shining over the earth, it has always evoked the imagination and praise of people and it is rendered more interesting due to the waxing and waning cycle making it grow and diminish in size over the lunar month.
The moon goes around the earth once in every 29.5 days. The moon cannot glow on its own and it just reflects the sun rays falling on it. During its various positions around the earth, it is exposed to varying amount of sun rays as the earth occurs in between. Therefore moon appears to grow and reduce in its size.
When the earth completely blocks the moon from the sun, it is Amavasya and when the moon is completely exposed to the sun, it is Purnima. Purnima is considered as a highly auspicious in Hindu tradition when the positive forces and divine energy rules over the earth. The birth anniversaries of some great souls are celebrated on Purnima including Vyas Purnima and Buddha Purnima.