The reason Easter is different is because it commemorates a single event in time that is connected not to days of the year or festive occasions but to a first day of the week resurrection- it must fall on a Sunday, hence the variations between calendars.
For me- it's a relatively easy question to answer: celebrate Pesach according to the Jewish calendar and other religious events according to the church calendar (neither calendar is "exactly right" anyway as both have changed a lot in history) This preserves the timing of both Pesach and the first day resurrection.
ContraMundum,
According to this link, Pope Gregory fixed the calendar problem by simply dropping ten days out of the calendar. On Thursday, October 4, 1582, the next day, Friday, should have been October 5, but Gregory made it October 15 instead, dropping exactly ten days to bring the calendar back into harmony with the heavenly bodies. Were the days of the week confused? No. Friday still followed Thursday, and Saturday still followed Friday etc. The same seventh day remained and the weekly cycle was not disturbed in the least. When we keep the seventh day on Saturday, we observe the same day Jesus kept, and Luke 4:16 tells us Jesus did this every week.
Here is a link for reference
Has The Calendar Changed The True Sabbath Day?
So, there has been no loss in the order of the days of the week. They have, by the account of all reputable theologians, remained in the same order since creation.
Okay, here is where I loose it:
"And ye shall count unto you from the morrow after the day of rest, from the day that ye brought the omer of the waving; seven weeks shall there be complete; even unto the morrow after the seventh week shall ye number fifty days; and ye shall present a new meal-offering unto the LORD." (Leviticus 23:15-16)
The Jews always start "Firstfruits" (morrow) the day after the sabbath onSunday, so "Firstfruits" is always on Easter Sunday. Jesus is mentioned in the New Testiment as the, "Firstfruits" quite a few times.
Notice that the days between the Passover and Firstfruits vary, and the Catholic Holy Thursday Seder is anchored to 3 days before Easter Sunday. Last year (2009) was a year when Passover did fall on Holy Thursday:
2009
Passover began at sundown on Thursday, April 7, 2009
Firstfruits began at sundown on Saturday, April 11, 2009
2010
Passover beings at sundown on Sunday, March 28, 2010
Firstfruits begins at sundown on Saturday, April 3, 2010
2011
Passover beings at sundown on Sunday, April 17, 2011
Firstfruits begins at sundown on Saturday, April 23, 2011
Matthew 27:51-54
1 Corinthians 15:20 --- 26