The Festival of Ostara is a celebration in Camelot where it is tradition to have a hunt that day.
It occurred shortly after Princess Mithian's arrival in Camelot. Arthur warned her, suggesting that Merlin arrange a tour of the city. However, Mithian told him that she loved hunting. Later on, while Arthur was hunting, Morgana enchanted Guinevere to appear as a deer so that she would be killed in the hunt, but Merlin was able to save her. The celebration was ruined as the hunt was cancelled because Arthur found the ring he gave to Gwen and realised he still loved her (The Hunter's Heart).
Ostara was the name to what now is known as Easter. In the old days and even in some Neo-Pagan, Druid, and Wiccan religions like wicca, Ostara is one of the eight sabbats celebrated each year. It falls on the Spring Equinox between March 20-Mach 22. It is vey common in the Pagan religion who follow the wheel of the year to Paint Eggs on Ostara to celebrate sping.