Possible Clues:

• Biannual event is mostly horse and bull
• Biannual celestial event
• Topping sequin socks from America for a special date
• Audre Lorde poem: a half-yearly occurence
• Time of equal night and day
• Occasion for gathering at Stonehenge
• When day and night have the same length
• Semiannual calendar event
• Queen in love embraced by old date in March?
• Vernal or autumnal occurrence