Adonis was the beloved of Aphrodite. During a hunting accident, he was killed by a boar. Aphrodite lost her mind over this, as any lover would. So, pleading and begging Zeus to return him, he compromised. Waving his Godly hand, or however them Gods there do it, *winks* he made it so that Adonis would spend half the year in the light with Aphrodite, and the other half in the darkness of the Underworld with Persephone. So today, in Ancient Greek calendar, Adonis’s travels would mark the mid point of the half year.
In Egypt, This would have been the New year, but also the wedding anniversary of our dearest Isis and Osiris. How neat is that?