"Gabriel's announcement to the Theotokos that she will conceive the Christ, and her ""Yes""
This is the Orthodox feast in the New Calendar or Revised Julian Calendar. This Calendar is adopted by some smaller churches in the Orthodox Tradition.
The majority (f.e. The Russian Orthodox Church follows the Old Calendar, effectively placing the feastdays 13 behind the common used Gregorian Calendar and the Western or Roman Catholic traditions).
Paschal feastdays are computed alike across the Eastern Orthodox traditions."