Possible Clues:

• Time when sun crosses equator
• Vernal --
• Semi-annual event
• Time of year when night and day are equal
• When day and night are the same length
• Like a horse without a tail, steer into biannual event
• Semiannual occurrence that begins 18- and 66-Across
• One beast like another with no tail in spring or autumn
• This time it's not finished being horsey before it is beastly
• It occurs in September and March