Hours which are past midnight are suffixed with next day date. In Panchang day starts and ends with sunrise.
The full moon day which falls in the middle of spring season in India is known as Vasanta Purnima. Vasanta Purnima is very significant day as it coincides with the festival of colors i.e. Holi. As nature displays various colors during spring, Holi which is also the festival of colors synergizes with nature at the same time.
It is believed that ritual of playing Holi with colors was started in the era of Lord Krishna. It was Lord Krishna who started playing Holi with Radha Rani and her female friends commonly known as Gopis. Vedic astronomy defines six seasons in a year and these seasons are known as Ritu. These six Ritu are-
Vasanta time in India usually starts on February 19 and ends on April 19. Hence all Hindu festivals which fall during this period are linked to Vasanta Ritu.
As Phalguna Purnima falls in the middle of Vasanta Ritu, it is known as Vasanta Purnima. It is important to note that Hindu festivals are not linked with seasons as all Hindu festivals drift away from seasons due to precession. As it takes thousands of years to notice seasonal change of Hindu festivals, it is assumed that festivals are linked to seasons and festivals are given alternate names based on the season they currently coincide with.
Vasanta Panchami is good example to understand that Hindu festivals are not strictly linked with seasons. Half of the time Vasanta Panchami is observed before February 19 i.e. much before Vasanta Ritu starts. Hence Shri Panchami, which is alternate name of Vasanta Panchami, is better way to name the festival. However Vasanta Panchami heralds the arrival of Vasanta Ritu as well as Vasanta Purnima.
Holi, which is observed during Phalguna Purnima, falls in middle of Vasanta Ritu. We can say that Holi is Vasanta festival and it would keep coinciding with Vasanta Ritu for thousands of years as drifting of festivals from seasons take thousands of years to make any significance difference.
Holi Purnima, Phalguna Purnima and Vasanta Purnima all points to the same day in Hindu calendar. The day of Vasanta Purnima is also the birth anniversary of Goddess Lakshmi and Saint Shri Chaitanya Mahaprabhu.