Are there standards for expressing dates akin to holidays? - date

Question

For example, Foo Holiday is the 3rd Tuesday in March. Bar Holiday is the 7th minute of the 7th hour of the 7th day of the 7th Month.
I have looked at standards like ISO8601 but it seems those are more concerned with explicit points in time.

Answer 1

As AlexApps99 described in their answer,
there is no standard that will cover all holidays.
For example, the start of Ramadan often depends on physically sighting the new moon, so it's impossible to predict the exact date, as cloudy weather might prevent anybody from spotting it in your country for a few days.
However, for many repeating events the follow simpler rules (e.g. Foo & Bar Holiday in your question), there is
iCalendar's RRULE (Recurrence Rule),
which is a standard for creating repeating patterns for recurring events.
Many calendar apps will have a GUI for creating RRULE.
RRULE is documented in
RFC 5545 Section 3.8.5.3.
For example, an event for U.S. Presidential Election day would be:
Every 4 years, the first Tuesday after a Monday in November, forever (U.S. Presidential Election day):
DTSTART;TZID=America/New_York:19961105T090000
RRULE:FREQ=YEARLY;INTERVAL=4;BYMONTH=11;BYDAY=TU;BYMONTHDAY=2,3,4,5,6,7,8
Example taken from RFC 5545 Section 3.8.5.3 Page 129
Foo Holiday (3rd Tuesday in March) could be described with:
RRULE;FREQ=YEARLY;BYMONTH=3;BYDAY=TU;BYMONTHDAY=15,16,17,18,19,20,21
Bar Holiday could be described with (7th minute of the 7th hour of the 7th day of the 7th Month) could be described with:
RRULE;FREQ=YEARLY;BYMONTH=7;BYMONTHDAY==7;BYHOUR=7;BYMINUTE=7
Unfortunately, the iCalendar RRULE spec can't can't handle more complicated rules.
Easter can't be done, (although https://github.com/sappjw/calendars contains Easter RRULEs accurate for
1900-2099, and the Python dateutil library's RRULE has an unofficial extension called BYEASTER, but even then, it only supports Western Easter).
Non-Gregorian calendars are also not supported, however, there is a
draft spec RFC7529 that adds
RRULE support for additional calendars, which would allow you to describe
Chinese New Year using the following syntax:
DTSTART;VALUE=DATE:20130210
RRULE:RSCALE=CHINESE;FREQ=YEARLY
SUMMARY:Chinese New Year
These define a recurring event for the Chinese New Year, with the
first instance being the one in Gregorian year 2013.
Example taken from RFC7529#Section 4.3.1

Answer 2

There's no standard way to represent holidays, because holidays are pretty hard to keep track of. Each country has their own set of holidays, and there are different definitions for what is considered a holiday (public holidays, school holidays, religious holidays, international hotdog day etc).
On top of that, the days that holidays lie on is pretty hard to predict. Many holidays celebrating events on the non-Gregorian calendar, such as Easter, will be on a different date each year, and others based on external events can be unpredictable into the future.
Many holidays are not just "days", but a range multiple days, or specific hours of a day, and you can't forget that these intervals have timezones attached too (and will be different in different countries)
With all this complexity, you'll need to create your own way to represent these holidays, probably ignoring some of the complexity you don't need to handle.
If you're sharing this data, a standard format such as iCalendar would be ideal, and you could store custom properties of the holidays in extension fields (starting with X-).
If you're just storing this data internally, you're better off making your own structure to store whatever you need, perhaps using ISO8601 to store the intervals.