diff --git a/.github/workflows/shell-drift.yml b/.github/workflows/shell-drift.yml
index 213e57c..df3860b 100644
--- a/.github/workflows/shell-drift.yml
+++ b/.github/workflows/shell-drift.yml
@@ -2,7 +2,7 @@ name: Check shared site shell
 
 on:
   schedule:
-    - cron: "23 7 * * 1"
+    - cron: "23 7 1,15 * *"
   workflow_dispatch:
 
 permissions:
diff --git a/docs/SPEC.md b/docs/SPEC.md
index d2f20dd..5922f10 100644
--- a/docs/SPEC.md
+++ b/docs/SPEC.md
@@ -416,7 +416,7 @@ The shared shell uses the main site's explicit navigation colours, including
 white labels and Close control in dark mode. A semantic check resolves one
 main-site commit and compares ordered accessible navigation links, footer
 columns and links, and the Menu breakpoint against both talks templates.
-Pages validation fails on drift or a fetch/parse failure. A weekly check
+Pages validation fails on drift or a fetch/parse failure. A twice-monthly check on the 1st and 15th at 07:23 UTC
 maintains one marked drift issue, skips unchanged reports, and does not create
 an issue when the source cannot be fetched or parsed.