1) Bissextile occurrence 2) Calendar rarity 3) February 29 4) February 29th 5) Intercalary event 6) Intercalary observance 7) March preceder, periodically 8) Quadrennial occurrence 9) The 29th, every so often 10) When Rossini was born in 1792 
[n] - the name of the day that is added during a leap year
bissextile day noun the name of the day that is added during a leap year