Ostara, according to Jakob Grimm in his Deutsche Mythologie, is the old High German name for the Easter festival. It is a plural; Grimm states that this is because the old festival lasted several days. A rough translation would be 'The Easters'.
Solstitial celebrations still center upon June 24, which is no longer the longest day of the year. The difference between the Julian calendar year (365.2500 days) and the tropical year (365.2422 days) continue to move the day associated with the actual astronomical solstice forward approximately one day in approximately every seven centuries.
Grimm also proposes that Ostara might have been the name for a Goddess, though he provides no direct evidence for this. Grimm may have based his goddess on the Anglo-Saxon goddess Eostre, as described by the Venerable Bede, who was also said to have a festival lasting several days. Bede indicated that this name was used in English when the Paschal holiday was introduced. This name was then converted to Easter, or Ostern in German.
Ostara probably shares a common root with the word "east", the direction in which dawn rises. It is mistakenly thought by some to be related to the words "estrogen" and "estrus". These words, however, are in fact derived from the Greek oistros, meaning "gadfly" or "frenzy".
Ostara is a modern Pagan festival, and there is no evidence that Spring Equinox festivals were called by this name in the past. According to Bede and Einhard, the month Eostremonat/Ostaramanoth was equated with April. This would put the start of 'Ostara's Month' after the Equinox in March. However, it must be taken into account that these 'translations' of calendar months were approximate as the old forms were predominantly lunar months while the new were based on a solar year. As a new moon, signalling the start of a lunar month, does not have a fixed date, it is impossible to equate Eostremonat/Ostaramanoth precisely with the Spring Equinox, which occurs at a relatively precise time and date.
Ostara is one of the eight major holidays, Wiccan sabbats or festivals of the wheel of the year, celebrated by largely Wiccan-influenced neopagan groups. It is celebrated on the spring equinox, in the northern hemisphere around March 21 and in the southern hemisphere around September 23, depending upon the specific timing of the equinox. Among the Wiccan sabbats, it is preceded by Imbolc and followed by Beltane.
The name is generally not used in British Traditional Wiccan traditions such as Gardnerianism, but Ostara, Eostra or other variants on that name are the most common names for the Sabbat in other modern Pagan witchcraft traditions.
The holiday is a celebration of spring and growth, the renewal of life that appears on the earth after the winter. In the book Eight Sabbats for Witches it is characterized by the rejoining of the Mother goddess and her lover-brother-son, who spent the winter months in death. This somewhat resembles the biblical story in which Jesus is resurrected, which Christians celebrate on Easter.