We are at sixes and sevens this year, because the feast of the Annunciation was trumped by Good Friday. When that happens, by Holy Week or the Easter Octave, the Annunciation is transferred to the first available day after the Easter Octave.
In the Roman rite and our ACC calendar, it was two days ago, Tuesday 5th April. In the Use of Sarum, we had St Richard, Bishop and Confessor on Sunday 3rd, but who was trumped by the first Sunday after Easter. His first available day was Tuesday 5th April. My Sarum Kalendar 2016, compiled by Dr William Renwick, gives today as the day for the Annunciation.
Everything comes to he who waits…. Be it done to me according to thy Word.
The Blue Flower 
And by an interesting coincidence – today is also the Annunciation in the Julian calendar.
How amazing, because I’m using the Gregorian calendar (as we all do in the ACC), and the Annunciation was deferred from 25th March (Good Friday this year) to Tuesday 5th April, and then to Thursday because Monday 4th (St Ambrose) wasn’t free, and St Richard (3rd April) had to be transferred to Tuesday 5th. Wednesday 6th was a Feria with proper Lesson and Gospel, so to the 7th went the Annunciation. Now you tell me that it coincides with 25th March in the Julian Calendar.
Indeed amazing!
It surely is amazing when the Julian and Gregorian calendars are on the same page!
In the EO world it is possible that Annunciation happens on Pascha, then confusion reigns and the rubrics require a slide rule to figure out! Divide the hypotenuse of the Annunciation by the circumference of Pascha and Lo! you have it….
I attend a Ukrainian Greek Catholic parish, which doesn’t move Feasts no matter what day they fall on. Therefore we celebrated Divine Liturgy for the Annunciation and observed Great Friday on the same day.
The idea that we should commemorate feasts on different days is sensible – like devoting a whole day to a concept. But that two concepts should coincide on the same calendar day is an inevitable result of feasts fixed on a calendar date and a theme fixed to a seasonally-determined day. We shouldn’t agonise over it. Simply anticipate that the clash will occur, prefer the seasonal theme – God over Mary – and make sure you commemorate the subordinate concept any convenient day before or after. God will not be worried.
Actually, if one follows the table of precedent; the combining or moving of feasts is not difficult (at least in the old rite). Of course, our resident expert, and a real expert on this by the way, is Rubricarius.