This is the first day of Anthesteria, a three-day festival celebrated in ancient Athens in honor of Dionysus. It was a celebration of the coming of Spring and the opening of the jars of new wine.
Today is the Pithoigia, on which the first jars (pithoi) of the new wine were opened, and libations offered to Dionysus. Celebrate with a glass of wine this evening, and offer some to the god.
Tomorrow is Choës, upon which the marriage of Dionysus was celebrated. Thursday will be Chytroi, a day to leave food and wine offerings for the spirits of the deceased ancestors.
The festival of Anthesteria combined religious ceremony, merry-making, an inversion of the social order along the lines of the Roman Saturnalia, and propitiation of the spirits of the dead.