Possible Clues:

• When day and night are of equal length
• Time of year when day equals night
• Time of equal length of day and night
• Autumnal --
• Semiannual astronomical event
• Time of year when night and day are equal
• Semiannual event
• Semi-annual event
• Time when sun crosses equator
• Like a horse without a tail, steer into biannual event