Ostara falls on the Spring Equinox, sometime between March 19-22 depending on the year. The Equinox is the day when the sunlight hours are the same length as the night. This means the day is a between time, which makes it a time of thinning of the veil.
This is the pagan holiday that lucky pagans can flaunt to Christians. The traditional symbols of the Christian Easter: the egg, the lily, the bunny, lamb and chick, are all symbols of the Spring Goddess. The very name Easter is a deviation of a Goddess name.
Ostara is the first day of Spring, and is a planting/sowing time. The first buds and sprouts of the year are coming up. New beginnings are now possible.
Main Themes of this Sabbat are new beginnings, doing divination for the year (as the veil is thin), doing ritual drama about Spring, egg hunts. A typical ritual could incorperate a drama about Persephone's descent and assention from Hades, emphasizing the assention, or one about the Japanese Goddess, Ukume. an egg hunt with divinatory symbols drawn on the eggs, and the charging of seeds, making it symbolic of some goal (as the plant grows, so too does the goal).