Eostar, Spring Equinox, March 20-23 dependent on actual astronomical event.
The Goddess blankets the Earth with fertility as the God stretches and grows to maturity. The hours of day and night are equal and light is overtaking darkness. This is a time of beginnings, action, planting spells, and of tending the gardens. This is the start in the pagan year of spring, at least among Wiccans and Celtics. The first flowers are praised and the God and Goddess thanked for the true return to happier times for all. Ostara is one of the more colorful holidays, not one of the somber colors found in Yule and Candlemas. Feasting and socializing are the important factors in this holiday as well as the celebration of the return of color to the natural world.
In the Christian calendar, again to draw early worshippers, they marked this as the final days and rebirth of Jesus (when according to history he died in June!)