The spring equinox — when the sun crosses the celestial equator heading north, and day and night are equal — falls around 20–21 March. From this point, days grow longer than nights in the northern hemisphere until the summer solstice. The equinox is the astronomical threshold of spring: not the first warmth (that was Imbolc) but the confirmed victory of light over dark, the point from which the bright half of the year extends.
The name "Ostara" comes from the Old High German Ôstara — recorded by the Venerable Bede in his 8th-century work De Temporum Ratione as the name of an Anglo-Saxon spring month (Ēosturmōnaþ) associated with a goddess called Ēostre. Bede is our only ancient source for this goddess; some scholars have questioned whether she was a widely worshipped deity or a localised tradition. What is not in doubt is that the name gives us both "Easter" and "Ostara" — and that the spring equinox has been marked with ceremony across cultures for as long as records exist.
Megalithic monuments aligned to the spring equinox sunrise include Loughcrew in Ireland and the Mnajdra temples in Malta. The Persian new year festival Nowruz — still celebrated by hundreds of millions of people — falls at the spring equinox and has been observed for over three thousand years. The Babylonian new year festival Akitu fell in spring. The spring equinox is one of the most universally recognised astronomical events in human history — the moment when the world's tilt means the sun rises due east and sets due west everywhere on earth simultaneously.