Easter is exactly when it should be…

Every year, I hear someone complaining about how Easter is too late, or Easter is too early. And the number of people that complain about how Easter can't seem to have a fixed date… Every year, I respond to these complaints in the same fashion.

"You do know how Easter is calculated, don't you?"

The blank stares are borderline hilarious.

Easter is always the first Sunday after the first full moon after the Spring Equinox.

(Okay, the Spring Equinox for the Northern Hemisphere.)

It's a simple calculation really, or at least one would assume so, until they discover the truth behind the equation. While the description appears to be one based on astronomical events, it's really not.

The Equinox Calculation

An equinox is the time of year when we have the same number of daylight hours as we do nighttime hours. However, the astronomical equinox can vary from the recognized date by up to two days every year. Notice I said recognized date. The equinox is nominally considered to be March 20th. However, for countries such as New Zealand, this date is actually the date before the astronomical equinox. For American Samoa, it's the date after.

So, Easter is some time after March 20th, depending on when the first full moon is after this date. Yet, this is another descriptor that appears to be astronomically related, but actually isn't.

The Paschal Moon

Easter is based on the Paschal full moon, which is the fourteenth day of the ecclesiastical lunar month. (Isn't that a mouthful to get your tongue around.)

A lunar month can be between 29 and 30 days, intended to approximate the observed phases of the moon. However, a true lunar month, based on astronomical data, varies between 29.27 and 29.83 days. (How confusing can we get?)

The Calculation for Easter

So, for the calculation of Easter, March 20th is the nominated date for the Spring Equinox, and the first full moon after this is some calendar date that is based on a bizarre calculation that approximates the lunar phases—but can get it wrong by up to two full days.

But Easter is always a Sunday.

Let's say that March 20th falls on a Friday, and the Paschal moon falls on March 21st. That means Easter can be as early as March 22nd. However, say the full moon before the equinox fell on March 20th itself. We then need to wait a full 29 days for the next full moon in the calculated sequence. As such, Easter can be as late as April 25th.

And if that isn't confusing enough, we have to consider the time of totality for the full moon within Greenwich Mean Time. In 2025, the first full moon after the equinox reaches totality at 00:22 GMT on April 13th (which would be 12:22pm NZT on April 13th, as New Zealand is GMT+12 at this time of year). April 13th, 2025 is a Sunday, meaning Easter is the following weekend, putting it on April 20th, 2025.

So you see, Easter is not early, and it's not late; it's exactly when it should be—even though the way they came to this date is incredibly confusing.

Random Lunar Facts:

Because the lunar cycle is approximately 29 days, it is possible to have two full moons in a month. The second full moon in that month is known as a Blue Moon. And they are more common than the phase "once in a blue moon" seems to suggest. Ironically, 2025 won't see a blue boon. The next blue moon will be on May 31, 2026.

But it should also be noted that because of the 29-day lunar cycle, occasionally, February will not have a full moon. When this happens, it's known as a Black Moon. The next Black Moon (month with no full moon) will be February 2037.

The term Black Moon can also refer to a month that doesn't see a new moon cycle. Because of how short February can be as compared to the lunar cycle, we will next see a Black Moon (month with no new moon) in February 2033.