Easter and Passover: Why do both fall on different days each year?

Ever wonder why Easter and Passover don’t fall at the same time every year? Sometimes it happens in March, at other times in April; sometimes — like this year — the two holidays overlap, while other years they can be nearly a month apart.
Most folks know when Easter and Passover are celebrated because they flip their calendars and see it written there, or, they read their church or synagogue schedules.
But, who tells the religious leaders and the calendar makers?
The quick answer is this: Easter is the first Sunday after the first full moon after the spring equinox. Passover, on the other hand, begins on the first full moon of Nisan, the first month of the Jewish lunar-based calendar.
But in reality, it’s a bit more complicated than that.

The basics

Easter is the Christian celebration of the resurrection of Christ, which is said to have occurred three days after a seder meal (The Last Supper) — that’s the connection to the Passover seder. Since Christ is said to have risen on a Sunday, that would make the day the meal took place a Thursday.
In Christian circles, the day commemorating this meal is called Maundy Thursday or sometimes Holy Thursday. Maundy is from the Latin mandatum, meaning command, because Jesus said on this day, “I give you a new command — to love one another.”
The celebration of Pesach, or Passover, however, is determined by the Jewish calendar, which is based on lunar months; each month begins on the new moon. Passover begins on the 14th day (that would make it coincide with the full moon) of the month of Nisan, also known as the first or spring-time month.

Lunar vs. solar

A problem began to arise with choosing a date for Easter because lunar and solar calendars don’t match. Therefore, many religious festivals began shifting into the “wrong” season.
In 45 B.C., Julius Caesar tried to tackle this problem with the creation of the Julian calendar. Among other changes to time-keeping norms, he added a day at the end of February every four years, thus creating leap year.
Before that time, however, in the early days of Christianity, calendar-keeping was done by the Jewish assembly known as the Sanhedrin; the method — a complicated time-keeping calculation based on a 19-year lunar cycle called the Metonic cycle — was kept secret.
Early Christians ran into another problem. Because Pesach was always on the 14th day of the month, using it to choose the date for Easter meant Easter would not always fall on a Sunday.
In 325, Emperor Constantine convened the Nicean Council in part to settle the dispute between those who thought the date for Easter should be calculated using the 14th day of the month method, and those who thought it should always be on a Sunday.

The council’s decision

The council decreed Easter to be the first Sunday after the first full moon after the spring equinox (March 21) — unless that full moon is on a Sunday, then Easter is to be celebrated the following Sunday.
Aside from being a set method of calculating the date of the holiday, this essentially did two things. It kept Easter on a Sunday, the original day of Christ’s resurrection. It also served to create a distinct separation between the Christians and the Jews.
It should also be noted that Constantine was said to be a sun worshiper. Setting the date of Easter in the manner he did may also have been done to include pagans, since the vernal equinox represents the rebirth of the sun.
So, Easter problem solved, right? Wrong.
Even with Caesar’s “every four years is a leap year” rule, the vernal equinox — the starting point for calculating Easter — began falling earlier than March 21.
This meant it was possible for Easter to actually come before Passover.
Pope Gregory XIII, in the 16th Century, helped with the problem of the drifting vernal equinox by adding to the leap year rule that the extra day in February would not be added during a year that is divisible by 100, unless it is also divisible by 400. This makes the average calendar year 365.2425 days — much closer to a true year (how long it takes the Earth to revolve around the sun) of 365.24219 days.
So now, Easter always falls between March 22 and April 25. However, because of differences in the solar or calendar year and the lunar year on which the Jewish calendar is based, Easter still occasionally falls almost a month before Passover — like it did in 2008. The next time it will do so is next year, in 2016, when Easter will be March 27, and the first day of Passover will be April 21.

Ecclesiastical vs. astronomical

But wait. There’s more.
For the scientific sticklers out there, it’s important to note that the dating of Easter is based on what is known as the “ecclesiastical” dates, not the actual astronomical ones.
The ecclesiastical equinox — the one set by the Church during the Council of Nicea — is always on March 21.
Astronomical calculations, which are far more accurate today than they were nearly 2,000 years ago, and changes to the calendar mean the vernal equinox — the point at which the sun crosses the celestial equator moving northward — sometimes falls on March 20.
Why should this matter when picking a date for Easter?
Take, for example, the year 2038. The actual vernal equinox will fall on March 20, and, there’s a full moon the next day. So, “astronomically speaking,” Easter should fall on Sunday, March 28.
But, “ecclesiastically speaking,” the equinox is March 21, and the church says Easter is the first Sunday after the first full moon, after the equinox. So, that year, Easter must come after the next full moon, so it’s not until the latest possible date: April 25.
OK, there’s one more thing. The church also uses an “ecclesiastical full moon.” Again, science was not quite as pin-point accurate as it is today, and so the church relied upon — and still uses — tables that were calculated during the early days of Christianity to predict when the full moon would be. While the tables are remarkably accurate, they don’t always coincide exactly with the astronomical full moon.
When it comes to figuring out when Easter is, it’s obviously not easy.
Unless, that is, one is content to simply look on the calendar.