Ostara is the spring celebration of fertility, rebirth, renewal, and new life. It occurs on the vernal (spring) equinox, which falls on or near March 21st in the Northern hemisphere.
Also called Eostre, it is the holiday from which Easter is derived, and shares the symbols of eggs and the rabbit. This is, obviously, due to the nature of birth and fertility associated with both symbols.
Ostara can be celebrated by constructing an altar, including objects such as spring flowers, eggs and eggshells, and candles in pale green, blue, yellow, and white (pastels and 'spring-y' colors in general are good.) Other rituals may include a maypole, making wreathes of suitable materials (spring flowers, eggshells, etc.) making offerings to the fae, and planting or renewing a garden.