Is it important that the exact day is celebrated or is it enough that people come together to reflect on...whatever you Christians reflect on.
The latter.
Scripture and Tradition go hand in hand, and Scripture is the 'last word' as it were, but it should also be remembered that the Tradition came first.
The way I read it.
The teaching was (from what we can glean) that Our Lord was crucified on the eve of the Sabbath observance, and rose from the dead on the day after, according to the data of Scripture.
Thus the Early Christian Communities celebrated the Eucharist on the day following the Sabbath, not just annually, but weekly; the Hebrew Sabbath is Friday-Saturday (dusk-dusk), so the Christian Sabbath would be Saturday-Sunday (dusk-dusk).
At first the ECC followed the Hebrew calendar. Growing tensions within the communities however, soon led to schism.
We can see in Acts that the Sanhedrin carpeted Peter and John over their preaching in the temple, and we can be pretty sure they put pressure on the community to ostracise the Christian blasphemers. James, a constant presence on the Temple (he was said to have knees like a donkey for the hours he spent in prayer) was killed, as was Stephen. Attempts were made on St Paul's life, and in the end it needed a considerable escort to get him out of the city, something done in the middle of the night to escape notice by the hotheads planning to kill him.
I very much doubt the two communities lived in peaceful co-existence.
In 60AD in Rome there were running street battles between Jewish and Christian mobs. Eventually the Jews forbad Christians from entering the synagogue.
The Christians responded by establishing their faith as distinct from Judaism. They have the Passover, we have the Resurrection.
It was a simple step, from there, to 'dispose' of the Hebrew calendar and fix a uniquely Christian 'stand-alone' event. Easter is still worked from the lunar calendar, but rather than the three days being determined according to the highpoint of the Jewish year, it's determined according to the highpoint of the Christian one — which was, by the then-established tradition, a Sunday.
For the Christian, the Resurrection is the thing: "And if Christ be not risen again, your faith is vain, for you are yet in your sins" (1 Corinthians 15:17).
The Christian Faith does not stand or fall on what day Nisan 14 fell in what year. It really doesn't matter. What matters, as our Orthodox brothers and sisters say, is that "Christ is risen! He is risen indeed!"
I think this practice was first established in Rome. In the east the communities continued to follow the Hebrew calendar for some while longer.
