Ostara is a spring equinox observance in the modern Pagan and Wiccan Wheel of the Year, and it is worth stating plainly that the festival is modern and the name is borrowed, because the two facts are usually blurred.
The Wheel of the Year, eight festivals at the solstices, equinoxes and cross-quarter days, was assembled in the mid twentieth century, principally through the collaboration of Gerald Gardner and Ross Nichols in the 1950s. No ancient culture is known to have kept all eight. The Irish sources give four quarter days without the solstices and equinoxes as festivals; Germanic and Norse sources give solstice observances; combining them into one eightfold scheme is a modern synthesis, and Ronald Hutton has traced the assembly in detail.
The name comes from Jacob Grimm, who in his Deutsche Mythologie of 1835 reconstructed a Continental Germanic goddess Ostara from the Old High German name for Easter and from Bede's Eostre. Grimm's reconstruction is a philological inference, offered as one; it is not a testimony. The goddess Ostara appears in no source.
None of this makes the modern festival illegitimate. A twentieth-century tradition that celebrates the equinox and calls it by a reconstructed name is doing what religious traditions do, and its practitioners are generally well aware of its recent origin, the scholarship on the subject was largely written by people sympathetic to it.
What is not defensible is the version circulated online in which Ostara is an ancient pan-European festival with an unbroken history.