NISAN 14

Attention! G-d DEMANDED in Ex 12:2 from His Chosen people (as known, for centuries Christians held themselves newly chosen instead of the Jews), the lunar month Nisan, in which started Exodus of G-d's People should be “the first month of the year”.
Thus G-d COMMANDED each family to select on Nisan 10 a lamb, which should be sacrificed on Nisan 14 and eaten roasted with bitter herbs on following night of Exodus on Nisan 15, but what remains should be burnt with fire. Doors of Jewish houses should have been stroken with the blood of the lamb in order the life of all the firstborn be saved when G-d would smite all firstborn in Egypt in houses not stroken with the blood of the Lamb. Further “an eternal ordinance” is set up to celebrate Pesakh (i.e. ‘coming over’ the stroken houses) during 7 days, of which no usual work is permitted on the 1st day, but a holy convocation, but nobody can eat leavened bread during 10 days (there was no time to leaven bread on the night of Exodus).
As for the Lamb and Sacrifice, all Christians agree this is Y-shua Messiah, the Saviour. However neither Y-shua Himself, nor G-d in general has ever revoked the ETERNAL commandment to celebrate on Nisan 14 in any special Revelation.
A QUESTION: what are grounds on which Christians from Gentiles do not celebrate the Resurrection of Christ during the days of Nisan commanded to them by G-d (if they really are on the place of the Jews today)? Who has revoked the eternal commandment of G-d then?
AN ANSWER: this one was even a not christianized pagan person Constantin, emperor of the Roman Empire, who “revoked” G-d's commandment in 325 in his letter to the first ecumenical Council of gentile Christians in Nicaea with approximately such words (an English translation from the Church History by Theodoret Cyrensis): “the purity of your minds may not be affected by a conformity in any thing with the customs of the vilest of mankind [i.e. the Jews]. /.../ we may have nothing in common with the usage of these parricides and murderers of our Lord .” 
No matter from whom that pagan person heard that the Jews had being “Deiciders”. However it does matter that CURSING of the whole Jewish nation (cf. Gen 12:3) in such a form was accepted by Christian BISHOPS (mostly Greeks) during the Council, which decided to celebrate Resurrection not in accordance with eternal G-d's commandment concerning Nisan 14. Differently from Constantin, the majority of those bichops could not but knew that there were THOUSANDS of Jews (Acts 2:41, 4:4, 21:20) in the First Church, and that they had been ousted from their fatherland by pagan Roman army after the first Judean War and the Revolt of Bar Kokhba. These Jewish Christians became dispersed outside the Empire but survived centuries in pure Jewish “heretical sects of Ebionites” (as said in Greek descriptions), not to say about those who coalesced with the Syro-Aramaic Church. The malediction of 325 fell on them as on Jews, but at the same time it automatically fell backwards also on the First Christian Church which represented the “nation of Dieciders” in Christianity.
The council of Nicaea had a formal justification in the absence of the Jewish Church with its bishops in their native land. However how then could they be present in own land while having being ousted from it by the Romans and (namely then) replaced with gentile colonists Greeks there? With such patriotic pro-Roman imperial logics namely Greeks (but not the Jews, enemies of the Empire) appeared to be the best legitimizers of the Sadducean Sanhedrin of Kaiaphas!
It is not clear how then Y-shua Himself, His Mother, a dozen of Apostles and first Jewish bishops succeeded to evade the malediction of 325! This is not our task to solve this mystery which is in he competence of the coming Last Judgement.
However it is in the matter of course that the malediction had practical sequels aimed to cut Gentile Christians from “the sacred root of the natural olive tree” (Romans 11:16–26): it was in 364 when a local Church Council of Laodicea prohibited them under ANATHEMA (!) to celebrate G-d's instituted eternal festivals, starting from Sabbath (even whilst they were Jews, not Gentiles). Nobody took care even of Christ's words in Matthew 24:28 (these words remained due to some muff in corporate censorship of the first two ages there, but were successfully removed from Mark 13:18). Luckily many Oriental churches kept to celebrate Nisan 14 during several centuries more thanks to the influence of St. John, but the sanctity of Sabbath still survives in the Orthodox church even today.