Vesak is the day the Buddhists regard as the most important one in their religious calendar. While the Theravadas call it by the name of Vesak, it has got different names, depending on the places where it is observed. For instance, in India, the land of Buddha, the Day is more familiar as the Buddha Poornima, the full moon of Buddha. It is called sa-ga zla-ba by the Tibetan sect and Vishakha Pujain in Sri Lanka.
So called, because the auspicious day falls on the full moon day. On the full moon day of Vaishakha, the lunar month that corresponds the month of April-May. The Day Commemorates the birth, enlightenment and death of the Buddha. So, the Buddhists consider the day to be more auspicious than any other day of the year.
In fact, it was on this day that Buddha's birth, enlightenment and death, or, entrance into final Nirvana, all happened to have taken place.
According to Buddhist legend, his wife Yashodhara, his first disciple Ananda, his charioteer Channa, and the horse Kantaka on which he renounced his kingdom to 'find some answers to life', were all born on this sacred day. Even, the tree, called under which he attained nirvana was also believed to have been created on this day. And it was on this day Buddha chose to preach his first sermon at Varanasi, or, Benaras, in India.
However, not all Buddhists commemorate the three events on this day. Though the events are commemorated by all.
It is only in the Theravada countries, the three events are all observed together on Vesak, the full moon day of the sixth lunar month Vesakha. This falls in the month of April-May.