Date rules for holidays: Easter, the nth weekday of a month, and the
days that replace a holiday on a weekend. The built-in countries use
them, and a Koyomi.Holidays.Provider can too.
Weekdays are numbered as in Date.day_of_week/1: 1 is Monday and 7 is
Sunday.
Summary
Functions
Returns Easter Sunday of year in the Gregorian calendar (the
Western churches' date), by the anonymous Gregorian computus.
Returns the nth weekday of a month, or the last one when n is
:last.
Returns the first weekday on or after date.
Returns the last weekday on or before date.
Returns the days that replace holidays on a weekend, as
{day, holiday} pairs.
Returns true when date is a Saturday or a Sunday.
Types
Functions
Returns Easter Sunday of year in the Gregorian calendar (the
Western churches' date), by the anonymous Gregorian computus.
Examples
iex> Koyomi.Holidays.Rules.easter(2025)
~D[2025-04-20]@spec nth_weekday(integer(), 1..12, weekday(), pos_integer() | :last) :: Date.t()
Returns the nth weekday of a month, or the last one when n is
:last.
Examples
iex> Koyomi.Holidays.Rules.nth_weekday(2027, 11, 4, 4)
~D[2027-11-25]
iex> Koyomi.Holidays.Rules.nth_weekday(2027, 5, 1, :last)
~D[2027-05-31]Returns the first weekday on or after date.
Examples
iex> Koyomi.Holidays.Rules.on_or_after(~D[2027-06-20], 6)
~D[2027-06-26]Returns the last weekday on or before date.
Examples
iex> Koyomi.Holidays.Rules.on_or_before(~D[2027-05-24], 1)
~D[2027-05-24]@spec substitute_days([Koyomi.Holidays.Provider.entry()], [ Koyomi.Holidays.Provider.entry() ]) :: [ {Date.t(), Koyomi.Holidays.Provider.entry()} ]
Returns the days that replace holidays on a weekend, as
{day, holiday} pairs.
Each holiday of eligible on a Saturday or a Sunday, in date order, is
replaced by the next weekday that is neither a holiday of holidays
nor a day already given to another holiday. This is the British rule
for substitute days: with Christmas Day on a Saturday and Boxing Day on
the Sunday, the days off are Monday 27 and Tuesday 28 December.
Examples
iex> christmas = {~D[2027-12-25], "Christmas Day", "Christmas Day"}
iex> boxing = {~D[2027-12-26], "Boxing Day", "Boxing Day"}
iex> Koyomi.Holidays.Rules.substitute_days([christmas, boxing], [christmas, boxing])
[{~D[2027-12-27], christmas}, {~D[2027-12-28], boxing}]Returns true when date is a Saturday or a Sunday.