How come Christmas can float, but Easter is fixed to Sunday? - Jinx 
Probably because of the way that Jesus died, was buried, and rose three days later on the Sabbath, which we now equate to Sunday.
Ah-ha! So, the day of birth is not as significant.
Luke, Chapter 23-24
(The burial of Jesus)
It was the day of Preparation, and the sabbath was beginning.
i.e., it was Friday.
On the sabbath they rested according to the commandment.
Saturday
But on the first day of the week, at early dawn, they came to the tomb. . . but when they went in, they did not find the body.
Mark, Chapter 16
And very early on the first day of the week, when the sun had risen, they went to the tomb.
Matthew, Chapter 28
After the sabbath, as the first day of the week was dawning, Mary Magdalene and the other Mary went to see the tomb…
John, Chapter 20
Early on the first day of the week, while it was still dark, Mary Magdalene came to the tomb and saw that the stone had been removed from the tomb.
Despite the differences in detail, it is one thing all four Gospels are specific about.
It is not a bad question. It sounds pretty goofy to have an extremely important historical religious date bounce around based on moon cycles.
Here is how it started but the reasoning is a little fuzzy:
“Prior to A.D. 325, Easter was variously celebrated on different days of the week, including Friday, Saturday, and Sunday. In that year, the Council of Nicaea was convened by emperor Constantine. It issued the Easter Rule which states that Easter shall be celebrated on the first Sunday that occurs after the first full moon on or after the vernal equinox. However, a caveat must be introduced here. The “full moon” in the rule is the ecclesiastical full moon, which is defined as the fourteenth day of a tabular lunation, where day 1 corresponds to the ecclesiastical New Moon. It does not always occur on the same date as the astronomical full moon. The ecclesiastical “vernal equinox” is always on March 21. Therefore, Easter must be celebrated on a Sunday between the dates of March 22 and April 25.”
It is actually even more complex than Shagnasty’s reply would indicate.
From the beginning, (or as early as we can get back), the Resurrection was celebrated each Sunday (which is how the Christian day of worship moved off Saturday and onto Sunday). Within a short period of time, there was a movement to celebrate the Resurrection with a larger and special annual celebration. Unfortunately, there was no clear indication of the exact date that it should be.
While the Last Supper and Crucifixion are tied to the Passover, the accounts of the Gospels are not uniform in describing the exact date on which the Last Supper occurred. Beyond that, the Passover is (in relation to the fixed Roman calendar of Julius Caesar) a movable feast, following the lunar/solar calendar of Judaism. A few people argued for a celebration connected to the Jewish Passover (which would result in the celebration being held on whichever day of the week happened to coincide with Passover) and still others argued that the Christians should deliberately avoid any Jewish connection, while still following a modified lunar/solar calendar in memory of the tradition of the Passover connection even while refusing to use the actual Jewish calculations. It was a fairly contentious discussion, with one 2d century pope even trying to excommunicate everyone at the East end of the Mediterranean for following the “wrong” date.
The Catholic Encyclopedia has a good article on the Easter Controversy although the article assumes knowledge of several arcane terms with which many Catholics would currently be unfamiliar, to say nothing of non-Catholics who have not happened to have studied history.
The difference between the Catholic and Orthodox reckonings is independent of the earlier decisions. Both groups continue to fiollow the same rules, but with the Gregorian reform of the calendar adopted in the West, the dates on which (liturgical) Spring are recognized (as opposed to the celestial reckoning of the equinox) has changed and so the calendars of the two groups now differ based on the Julian or Gregorian reckonings.
The Orthodox do have one additional rule – Easter can never fall on or before Passover.
No, it’s not. Birth is something that happens to every animal, human or not. Rising from the dead is not nearly so common.
Christmas came from a mere mid-winter rite. Easter celebrated rebirth and spring, which was always more important to agrarian communities.
That is, not only do Christians always celebrate Easter on Sunday, but Easter is the whole reason Christians celebrate Sunday on Sunday.
Just to make sure that everyone is on the same page here. The day given for Jesus’s rising was given as Sunday which is the day after the Jewish Sabbath. As already noted, some (but not all) Christians have their holy day on Sunday. Even among those who take Sunday off, some still use “Sabbath” only to mean the Jewish Sabbath in respect to the people who defined the term.