diff --git a/tests/Service/Rtt/SolidarityDayResolverTest.php b/tests/Service/Rtt/SolidarityDayResolverTest.php
index 215c1a6..388c0a0 100644
--- a/tests/Service/Rtt/SolidarityDayResolverTest.php
+++ b/tests/Service/Rtt/SolidarityDayResolverTest.php
@@ -35,5 +35,29 @@ final class SolidarityDayResolverTest extends TestCase
         yield '2025' => [2025, '2025-06-09'];
 
         yield '2026' => [2026, '2026-05-25'];
+
+        // Century-boundary year: Easter 2000-04-23 → Whit Monday 2000-06-12
+        // (verified with: easter_date(2000) → date('+50 days'))
+        yield '2000' => [2000, '2000-06-12'];
+
+        // Late-April Easter (2011-04-24) → Whit Monday 2011-06-13
+        // (verified with: easter_date(2011) → date('+50 days'))
+        yield '2011' => [2011, '2011-06-13'];
+
+        // Easter on April 25 — exercises the computus corrective $m branch:
+        // Easter 2038-04-25 → Whit Monday 2038-06-14
+        // (verified with: easter_date(2038) → date('+50 days'))
+        yield '2038' => [2038, '2038-06-14'];
+    }
+
+    /**
+     * The returned date must always be a Monday (ISO weekday = 1).
+     * Verified for 2025 as a representative case.
+     */
+    public function testPentecostMondayIsAMonday(): void
+    {
+        $resolver = new SolidarityDayResolver();
+
+        self::assertSame('1', $resolver->pentecostMonday(2025)->format('N'));
     }
 }