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As the calendar flips over to March this weekend, take a minute to contemplate this: This Easter, on the 23rd, will be the earliest Easter in the entire lifetimes of everyone living on the planet today.

In fact, it's the earliest for any year in the 250 years between 1875 and 2124. It is matched only by Easter in 1913 - 95 years ago. So, for a few of our oldest neighbors (my mom among them), this will actually be the second time they've enjoyed an Easter this early.

Under the ecclesiastical rules set centuries ago by the Roman Catholic Church, Easter can fall anywhere between Mar. 22 and April 25, according to calculations by M.J. Montes.  But some dates crop up on the list more frequently than others.

For example, in all of those 250 years, Easter never occurs on Mar. 22 - the earliest date possible. It occurs only once on Mar. 24. That was in 1940 - the rarest Easter date of them all in that quarter-millennium. Easter falls on Mar. 23 only twice (in 1913 and 2008) and just twice on April 24 (in 2011 and 2095). All the rest are more common than this year's Easter date.

The most common dates for Easter between 1875 and 2124? Those would be April 10 and April 17 - with 11 Easters each on those dates.

So what are the rules for setting the dates for Easter?

The shorthand answer has always been that Easter falls on the first Sunday after the first full moon after the Vernal Equinox. This year, the Equinox occurs on Mar. 20, the moon is full on the 21st, and so Easter lands on the 23rd.

The actual rules are a bit more arcane. The quirk is that Church fathers did not adopt the astronomical definition for the Vernal Equinox, which can shift across several dates centered on Mar. 21. Instead, the Church fixed its ecclesiastical equinox on Mar. 21.

They also established "lunation" tables to determine when the full moons occur - tables that track the astronomical full moon, sort of, but not always, according to the U.S. Naval Observatory.

As a result, the ecclesiastical equinox can differ from the astronomical, and the ecclesiastical full moon can stray from the astronomical definition. To make it even more confusing, various branches of Christianity, in particular the Eastern churches that still use the old Julian Calendar, follow their own rules, producing different dates.

The rest of us just check the calendar. Happy Leap Day.

UPDATE: An earlier version of this post mistakenly listed April 25 as another date on which Easter does not occur between 1875 and 2124. In fact, it occurs three times in that period: 1886, 1943 and 2038. The WeatherBlog regrets the error.