# Calculating Easter CHALLENGE

Hello, I’m fairly new to Facer and am trying out different things. I am working on a USA Holiday watch face where the Image will change on the day of the holiday. When there is no holiday, it is fairly basic with some graphical movement, to keep the interest. Here’s my issue. Easter does not fall on a specific date, rather it lands on the First Sunday after the Full Moon after the Spring Equinox. Equinox, from what I have read, and based on the Gregorian calendar, falls on March 21st of every year. Then the next full moon is used to put a date on Easter. For example, the first full moon after March 21st, 2024, is the 25th. The 25th is a Monday. Easter would fall on the following Sunday which is March 31st. I want to know how to calculate Easer in Facer using the built-in Moon Phase widget, if possible, but have it work every year seamlessly. I’ve looked at how to get the Moon Phase, but nothing pops up on how to actually calculate the date for Easter. Thank you in advanced.

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Ha Ha Welcome @wingr.rew Fairly new to Facer wants to make the Most complex Formula . Besides what you have outlined . Next year is a leap year . If you wanted your Calendar to last for Millenia you would have to take into account there being an extra Leap every 100 years and 10000 . Any way it will be a little easier than trying to have a Calendar to tell when the Clocks go back to standard time .
Over here ( UK ) the main Celebration of Easter is on a Sunday .
I personally will not be Volunteering for this one . I have used the Christmas and Halloween formula on Faces . I can show you that for Fun .

There are some Sleeping Giants of Mathematics . They are not always easily awoken . I could Invoke them but I always feel that is rude .
They I am sure given a bit of Time may accept the Challange . Often the Magic word is Please .
Some times , as you see I modified your Title , offering a Challange is enough .

BTW the Winter Solstice here was called on the 21st however the Equinox was called on the 22nd . I don’t know if the Equinox is ever called like that .
I think they built Done Henge to try End the Argument . We will leave the Wobble Out .

This is formula for counting down to Halloween . I chose this because it is a bit easier to see which numbers you change . There is a useful Tag (#DD#) in Facer that tells you the day number for that day in that year . Useful for testing and building formulas
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`(\$(#Dy#/4)==floor(#Dy#/4)&&#DD#<=305?305-#DD#:\$\$(#Dy#/4)!=floor(#Dy#/4)&&#DD#<=304?304-#DD#:\$\$((#Dy#+1)/4)==floor((#Dy#+1)/4)?365-#DD#+305:\$\$(#Dy#/4)==floor(#Dy#/4)?366-#DD#+304:365-#DD#+304\$)`

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Well, we now have a leap year tag (#DISLEAPYEAR#), so maybe the formula becomes easier…

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Hey guys, I don’t think Easter depends on leap year at all. As I mentioned, Easter is calculated to be the FIRST Sunday AFTER the FIRST Full Moon AFTER the Equinox, which is March 21 no matter what year, or whether or not there is a leap year. Someone correct me if I’m wrong, but that’s the way I understood what I read in multiple websites.

The simple standard definition of Easter is that it is the first Sunday after the full Moon that occurs on or after the spring equinox. If the full Moon falls on a Sunday then Easter is the next Sunday.

Snippet from timeanddate.com:

## How Is Easter Determined?

Easter falls on the first Sunday after the Full Moon date, based on mathematical calculations, that falls on or after March 21. If the Full Moon is on a Sunday, Easter is celebrated on the following Sunday.

Although Easter is liturgically related to the beginning of spring in the Northern Hemisphere (March equinox) and the Full Moon, its date is not based on the actual astronomical date of either event.

• March 21 is the Church’s date of the March equinox, regardless of the time zone, while the actual date of the equinox varies between March 19 and March 22, and the date depends on the time zone.

• The date of the Paschal Full Moon, used to determine the date of Easter, is based on mathematical approximations following a 19-year cycle called the Metonic cycle.

Both dates may coincide with the dates of the astronomical events, but in some years, they don’t.

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Because Easter is after the end of February in each year and the number of days in February changes according to the Leap years it us a good Idea to take it into account . You can not bother and it could be correct for this year but it would be at least a day out for the year after .

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With all due respect, I think you are over thinking it. Here is a table of when Easter Sunday falls from year 2024 to 2030. I compiled it using March 21 as the fixed Equinox (per the statements I replied earlier). MARCH 21st of ANY year is considered Equinox. Then Easter is after the FIRST full moon AFTER March 21st (Leap Year does not come into the picture). I checked the dates, and they are all correct.

Year Equinox Full Moon After Equinox Easter Sunday
2024 3/21 3/25 3/31
2025 3/21 4/13 4/20
2026 3/21 4/2 4/5
2027 3/21 3/22 3/28
2028 3/21 4/9 4/16
2029 3/21 3/30 4/1
2030 3/21 4/18 4/21

I do have to add, these are for USA, not sure about anywhere else.

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Unfortunately it is not that easy. The table may seem straight forward, but Facer does not have a programming language, so we are limited.

To find a universal formula for the above table brings about challenges as @russellcresser pointed out. Just some points to consider:

• The day of the year varies for the equinox. (Tag #DD#)
It is the 81st day when we have a leapyear and the 80th day of the year when not.

• Then we face an issue on the 31st. of March 2027. Facer moon value for that day is Full Moon. So this calls for an additional exception. (Tag #MOONPHASE#)

• The moon phase on Easter Sunday varies. It is either Last Quarter or Waning Gibbous

So, just looking into all this, to be honest, it is why I have not bothered with the math around this. And probably why others haven’t done this either.

The “cheap” way would be to create 7 Easter images and check for the Easter Sunday date.

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I have looked into the equations for calculation of the dates and find it easier to add like 10 layers, each with its one condition for pre-calculated date (and repeat after 10years if needed), than trying to build one universal formula. I was not so much bored yet to be willing to dive into this.

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Tom, thanks for clearing that up. I wasn’t thinking that the #MOONPHASE# tags would be that far off, but I suppose if it is using the actual moon phase, which could be different each year, it would make a difference in any calculation, including leap year. I may do what you and petruuccios suggested. Thank you to all.

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A quick-and-dirty solution can be to pre-calculate manually the easter for this year. Of course it will fail for 2025 but you can easily update once-per-year this and re-publish your face.
May be if in the future there’s the possibility of having such as a “scripting language” (with variables, flux control, etc.) it is feasible to use a multi-year solution, but right now is a clear over-engineering problem.

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