"Gabriel's announcement to the Theotokos that she will conceive the Christ, and her ""Yes""
This is the Orthodox feast in the Old or Julian Calendar. The majority of the Orthodox churches accross the world still follow this calendar.
This calendar didn't drop the days to correct the error due to a too short year 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."