Possible Clues:

• Time of year when day and night are equal
• See 13-Down
• Autumnal - -
• Vernal - -
• When day and night are equal
• Biannual phenomenon affecting horse that's tailless, and castrated bull
• March time
• When the sun crosses the equator
• Time of year when the Sun crosses the equator
• Time of year seeing one of five bison, perhaps suppressed by Spain