Cleopas, as a former Seventh Day Baptist, I am somewhat familiar with Protestant Passover observances as well as those of Armstrong's break off groups and the so-called Messianic Jews.
Today, those who falsely call themselves Jews (whom Christ calls the "synagogue of Satan" [Rev2:9]) observe their Passover on the 15th of Nisan. According to the Law of Moses, it is supposed to be observed on the evening of the 14th of Nisan. The 15th being of course the first day of the Feast of Unleavened Bread.
2000 years ago, some observed the Passover on the night our Lord did (the evening ending the 14th) whilst many others were already observing the 15th. As you yourself already acknowledged, both practices were quite acceptable amongst the Jews of that time.
However the Holy Apostles forbid Christians (the true Jews) from celebrating the Pascha with the false Jews. Even the Quartodecimens remembered the Lord's betrayal on the 14th and did not celebrate His Holy Ressurection until the 17th thus avoiding celebration with the unbelievers.
The Orthodox Church and the Latin/Roman Catholics use the SAME method to calculate the date of Pascha. The difference in timing occurs because Rome change the calendar. Hence, on the Gregorian Calendar the 21st of March false 13 days earlier (for the moment at least). So then the first full moon after that date may or may not be the same date for us. This year it is not (obviously). The trouble with Rome's calendar change in this respect is that it causes them to end up celebrating the Holy Pascha before the Jews and so may well end up celebrating with the Lord Christ's murderers. Orthodox Christians do not do this.
May I invite you to come and celebrate the Holy Pascha
this year in the New Jerusalem? The Holy Bible clearly says that The Church is the New Jerusalem:
He that hath the bride is the Bridegroom (St. John 3:29).
Ephesians 5:22-33 makes it clear that The Church is the wife of Christ.
When an angel wanted to show St. John the wife of Christ the Lamb, he was shown the New Jerusalem:
And there came unto me one of the seven angels which had the seven vials full of the seven last plagues, and talked with me, saying, Come hither, I will shew thee the bride, the Lamb's wife. And he carried me away in the spirit to a great and high mountain, and shewed me that great city, the holy Jerusalem, descending out of heaven from God, (Revelation 21:9-10).
Hence, the Church is the New Jerusalem. Make sense mate?
You are welcome to come and celebrate here as soon as you are baptised and chrismated in the holy true and ancient Orthodox Christian Faith which was once delivered to the saints.
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buzuxi, peace be with you.
With all due respect I disagree. The Coptic Rite of Passion Week states that Palm Sunday fell on the 10th of Nisan which makes Thursday the 14th and Friday the 15th.
The former Pascha used to refer to the whole time from the day of Passover until the last day of the Feast of Unleavened Bread. This whole period was refered to as the feast. Acts 12 makes this clear by saying that the days of Unleavened Bread had begun yet Herod wished to wait until after the Pascha feast. If the Pascha meant only the day of Passover then it was over and there would have been no need for him to wait any more. The issue mentioned to Cleopas above about when they observed Passover in that time may clarify things a bit. However, considering how many other laws they broke (eg. holding a trial at midnight &c.) what would stop them from condemning the Lord in the night before they were to observe the Passover? Do not the Gospels state that they would not go to a certain place least they be defiled and made unable to eat the Passover?
I agree with your last paragraph my friend up until "...the Passover no longer to be celebrated." Well done with all you say in this respect.
Not all Protestants use unleavened bread. Indeed, the Primative Baptists make a point of using unleavened bread whereas most Baptists use leavened bread. When I was an SDB a disagreement amongst the local congregation meant that both were made available on the eve on Nisan 14th for us. I used to take the unleavened bread and this is one of the issues I struggled with coming to the Orthodox Christian Faith.
Also, correct me if I'm in error please but I believe our beloved Ethiopian brethren celebrate with unleavened bread only on Thursday of Passion Week. Is there something telling in this?
Personally I'm still unsure of whether the Lord took leavened or unleavened bread that night but I do realise that leavened bread was not forbidden until the next day so He may well have used this. I understand the spiritual significance of both.
When did the Armenians switch to unleavened bread? Was it not about the 6th or 7th century?
Is anyone able to provide historical evidence of when Rome made the switch also please? Who was the Roman Pope and exactly when did they change? If a reason was provided that would be nice too please.
On another note, 1st Corinthians 5:7 says that Christ our passover
is sacrificed for us (KJV). The DRB ends the verse, "For Christ our pasch is sacrificed."
The use of the word "was" somewhat denies the eternal nature of the Mystery of the Lamb slain from the foundation of the world (Revelation 13:

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Thank you for reading. Peace be with you and pray for me please.