Recipes — practical forms of the schedules long said to be unwritable

Translated from the canonical Japanese page recipes/README.md. The source_sha above records the source revision; a consistency check flags this page when the Japanese original changes.

Schedule requirements that cron, Quartz, and iCalendar RRULE have long been said to be unable to express, presented as one page per requirement. Each page runs “what happens today (with sources) → the Kairos expression (execution-verified) → try it in your browser” — the answer first, the background after.

Page	Requirement
Last day of the month	cron’s most common stumbling block, up to the last business day
Easter	The canonical movable feast RRULE cannot express — pure arithmetic, zero data
What is a 4-4-5 calendar?	Definition, the 4-5-4/5-4-4 variants, the 53rd week, a 2026 period table; the carry rule falls out of the definition
Nearest weekday to the 15th	The general form of Quartz `15W`, extendable to holidays
US Election Day, Black Friday and the Sunday after Thanksgiving	Dates relative to a computed date; RRULE’s `BYMONTHDAY=2..8` encoding written in the order the law states it
Patch Tuesday	The second Tuesday at 10:00 Pacific, shown in any time zone from one definition; why “second Wednesday, 3 am” in Tokyo is wrong in 10 of 12 months

Every code example is execution-verified by the reference implementation’s doctests (impl/test/doctest.test.ts). # eval: gives the evaluation range and #=> the expected output (conventions: reference). Blocks using @JP get the standard doctest premise (Gregorian, Asia/Tokyo, wkst: Mon, with a calendar entity carrying Japan’s confirmed 2026 holidays).
Each page’s Playground link is a self-contained form (premise included) — run and edit it directly in the browser (the Playground executes the reference implementation as is; nothing is sent to any server).
The full catalogue of collected “unwritable” cases (32 items with sources) and their three-way classification: study 11. These recipes are the practical re-arrangement of that study’s measured section.
The language itself: specification · vocabulary: descriptor reference.