I waited a few days to post to see if anyone offered a response to your two inquiries. I will respond to this one, but your other post, the eschatalogical question about Israel, is better left to Ariel, Athol, or someone else entirely :-).
I find the whole calendar issue can get very complex, so I am going to try
to keep this relatively simple in relation to your question. If this doesn’t answer your question, let me know.
In the Book of Exodus, Chapter 12, we find that the dates and ritual elements of the Passover in Egypt are set by the Lord Himself (i.e., “The Lord said to Moses and Aaron...”). The Lord said, “On the tenth of this month every one of your families must procure for iteself a lamb...” “You shall keep it until the fourteenth day...” The month is the month of Nisan.
There is no corresponding Scriptural command for when or how to celebrate Easter, the passover of Jesus Christ from death into life. Though we do know, of course, that the resurrection of the Lord was celebrated in every Sunday celebration of the Mass. In fact there was some controversy about the date of Easter in the early Church and no real consistency until the 4th century. So when you say that “Christians say they are following the true dating requirements, I am not sure what you (or they perhaps) mean.
In the first place, the controversy regarding the date of Easter centered on the fact that 14 Nisan could fall on any given day of the week. The Church wished to associate the celebration of Easter (the new Passover of Christ from death to newness of life) not only with the original Passover, but with Sunday, the day on which the Lord rose from the dead. In addition to this issue was the issue of which Sunday on which to celebrate.
It was the Council of Nicea which actually spoke to these issues. It fixed the date of Easter throughout the Church, stated that the celebration should be on Sunday, and stipulated most pertinently that Easter Sunday should be celebrated on the Sunday following the first full moon following the vernal equinox (unless that full moon fell on a Sunday, in which case Easter would be the following Sunday).
So given this, you would expect that Easter would fall no more than a week from the start of Passover. Sometimes, however, the difference can be more, with Passover even falling later than Easter. This is where the “leap” issues you refer to come in. I'm not going to go into this in detail because it makes my head spin. But it has to do with the difference in the Jewish calendar which uses lunar months and the Gregorian calendar which really abandons any correlation of the action of the moon in defining months. Bottom line is that every so often the Jewish calendar has an extra month (13) to account for a shift in the calendar in relation to the seasons of the solar year (this is important because Passover is described in Torah as a festival of spring). The result, for example, could be seen last year when Easter fell on March 23 on the Catholic Church calendar and Passover began on April 20.
The Orthodox use a Julian calendar instead of the Gregorian (as the Catholic Church does) and therefore we do not usually celebrate Easter on the same day (though it can happen). A further difference occurs because the Western Church uses a fixed date for the vernal equinox (March 21) while the Orthodox still use, I think, actual observances of the full moon. In 2009, for example, Easter will be celebrated by Catholics on Sunday, April 12 and by the Orthodox on Sunday, April 19. There's actually a Catechism paragraph related to this difference. It is 1170
Who has the correct date? Does the date really matter as long as the intent is there?
So what now is the question? Should Christ’s Passover from death into life be celebrated on Sunday rather than begin at sundown on 14 Nisan? I suppose that would depend upon the intent and what you are celebrating.
In Dies Domini, Pope John Paul II reminds us, “The Lord's Day — as Sunday was called from Apostolic times — has always been accorded special attention in the history of the Church because of its close connection with the very core of the Christian mystery. In fact, in the weekly reckoning of time Sunday recalls the day of Christ's Resurrection. It is Easter which returns week by week, celebrating Christ's victory over sin and death, the fulfilment in him of the first creation and the dawn of "the new creation." He also notes: “The fundamental importance of Sunday has been recognized through two thousand years of history and was emphatically restated by the Second Vatican Council: "Every seven days, the Church celebrates the Easter mystery. This is a tradition going back to the Apostles, taking its origin from the actual day of Christ's Resurrection — a day thus appropriately designated 'the Lord's Day'." It seems fitting that we celebrate on Sunday.
Also as Christians are we allowed to participate in Jewish Passover?
This there are some posts on. Sure, I would say. See Athol’s original post and subsequent items on Passover seders.