When Advent 4's own Sunday lands on Dec 24, the Vigil of Christmas's
identity (rank, collect, Lauds antiphons, Matins Gospel nocturn) now
governs the day, matching the reference engine's own rule that a
privileged Sunday never displaces it. Unlike the reference engine's own
"no commemoratio" for this exact collision, Advent 4 is deliberately
still commemorated — the Sundays of Advent have real standing of their
own. Matins' three-nocturn Sunday psalmody is untouched either way,
since it's driven by weekday, not by which id wins.
Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01ME8QNPdEmHSbSx1r6VmDRG
