Old Icelandic Calendar (Misseristal) (古冰島曆)

The OLD ICELANDIC CALENDAR — the Norse reckoning that never died: Iceland still takes a PUBLIC HOLIDAY on sumardagurinn fyrsti (the First Day of Summer, opening the month of Harpa), still gives husbands bóndadagur (Þorri's first day — the þorrablót season) and wives konudagur (Góa's first day). The engine runs on ONE windowed rule — the First Day of Summer is the first Thursday after 18 April — with everything else a fixed offset in the 30-day month chain: bóndadagur = FDS − 90, konudagur = FDS − 60, the First Day of Winter = the next FDS − 180. The structure is a theorem factory (every month opens on a fixed weekday because 30 ≡ 2 mod 7; winter is exactly six months; bóndadagur is always the 13th week of winter, the tradition's own formula) — and it DEBUGGED ITS IMPLEMENTER: two draft window-rules refused to boot against the ring laws, exposed by the leap-year case (bóndadagur 2024 = 26 January, which no naive January window produces). Anchored to the published living dates: 2025's Jan 24 / Feb 23 / Apr 24, 2026's Jan 23 / Feb 22 / Apr 23, and the First-Day-of-Winter pair 2023-10-28 (the late case) and 2024-10-26. Any date returns its misseri (skammdegi/náttleysi), old month with meaning, day, week-of-season, the aukanætur and sumarauki flagged, and the year's four key days. The Norse tradition joins the catalogue. Pure computation, no LLM.

Cost: 2 TIAN Points per call. tRPC procedure icelandic.date.