Annunciation to the Virgin is one of the most important feasts in the Syriac Orthodox Church, so important that the Holy Liturgy (Qurbono) must be held on this day even if March 25th also happens to be Good Friday! (The Holy Liturgy is otherwise prohibited on Good Friday since the Church celebrates the death of Christ; in fact, the altar is stripped of all altar vessels and covered in black on Good Friday.)
The Holy Friday according to the Gregorian calendar falls on March 25th in 2005, the same day the Church celebrates suboro, the feast of the annunciation to the Virgin Mary. According the Hudayo canon, "That day is the basis of all moronoyo feasts (feasts of the Lord)." Chapter V, 1:5. It is mandatory that the Holy Qurbono be offered on that day.
The Holy Synod of the Syriac Orthodox Church in Malankara has decided that the Holy Qurbono should be offered in all Malankara parishes on Holy Friday of 2005 prior to 8 am. The decision was announced by the Synod Secretary, Mor Gregorius Joseph. The Episcopal Synod cited the guidelines established in the Encyclical No 464 (dt. 24 Mithunam 1938) by the late Patriarch Mor Aphrem I Barsoum, which directs that the Holy Qurbono be offered prior to the Hasho services of the Holy Friday when it falls on March 25th.
The last such instance was on April 7, 1950 when the Malankara Church followed the Julian calendar. The Syriac Orthodox Church in the Near East and its diaspora observes the Holy week according to the Julian calendar and hence does not have a similar circumstance
Suboro - Annunciation to the Virgin -: http://sor.cua.edu/Feast/Annunciation.html