[Tagging] Opening hours syntax for non Gregorian calendar

[Kevin Kenny]

On Wed, May 22, 2019 at 10:10 AM Simon Poole <simon at poole.ch> wrote:
> That is not really correct as written, OH has the concept of variable dates which are based on some external definition of when they exactly are, currently the only one defined is "easter".  Typically you would use these to start/end date ranges or a single date that is on or can be defined relative to such a date. So adding "ramadan" or any other, externally defined date of note is not really an issue as long as the string used doesn't conflict with anything else.

'easter' suffices for the entirety of the Christian calendar[1].  All
of the movable observances, from Septuagesima to Corpus Christi
(including well-known ones such as Ash Wednesday, Good Friday,
Ascension Thursday and Pentecost) are specified as a particular number
of days before or after Easter.

I'd be fine with adding such things as Ramadan, Eid al-Fitr, and
Muharram to the schema. Since it's easiest to consider fixed points of
the calendar, Hilal ar-Ramadan would most likely be the anchor point
for Laylat al-Qadr. Eid al-Fitr would have to be a separate point,
since it's determined by its own astronomical observation. (Ramadan
runs an extra day in most localities if clouds prevent the observation
of Hilal as-Shawwal.)

[1] Strictly speaking, the Feast of St Leander - a minor observance -
also is variable, observed on 27 February in common years and 28
February in leap years. This inconsistency arises because in the Roman
calendar, the days counted from the *end* of the month, and Leap Day
was done by repeating the _sextilis_ of February. (A leap year may
still be called a 'bissextile' year.) As far as I know, he's the only
saint on the modern calendar to have been martyred in the last five
days of February of a leap year. But I don't know of anything whose
opening hours are tied to the Feast of St. Leander. There's also a
complicated set of precedences and special cases for when movable
observances collide with fixed ones (e.g. what happens when Good
Friday falls on the Feast of the Annunciation), or when certain fixed
obervances fall on a Sunday, but I haven't heard any call to model
those, either.