[Tagging] Orthodox Christianity exists | Re: Opening hours syntax for non Gregorian calendar

[Kevin Kenny]

On Fri, May 24, 2019 at 8:16 AM Rory McCann <rory at technomancy.org> wrote:
> On 24/05/2019 02:44, Kevin Kenny wrote:
> > 'easter' suffices for the entirety of the Christian calendar
>
> Nope! 1,000 years ago the Christian church(s) had the "East West
> Schism", resulting in Orthodox Christianty in the east, and
> Catholic/Prodestant Christianity in the west. The 2 sides often disagree
> about when Easter is.
>
> https://en.wikipedia.org/wiki/Template:Dates_for_Easter
>
> As far as I can tell the `opening_hours` syntax on the wiki doesn't
> specify which easter, but I presume it refers to the locally observed
> easter.

You're right, I oversimplified.

In the Byzantine church calendar, the fixed observances are tied to
specific dates, and the movable ones are tied to the date of Easter,
so the same assertion applies that Easter is the only date that must
be specified separately.

Julian Easter is a different day about 2/3 of the time. Not only are
the calendars 13 days apart, but the algorithm for calculating the
date of Easter uses a different approximation to the orbit of the
Moon. Aside from that, for the next 80 years, Julian-calendar fixed
holidays are at a constant offset from Gregorian-calendar ones. Few
sets of 'opening_hour's will have that sort of longevity.

And I'm fine with having julian_easter be a separate specification.