diff --git a/wizard-work/beam/test_zeller_dow.py b/wizard-work/beam/test_zeller_dow.py
new file mode 100644
index 0000000..4643013
--- /dev/null
+++ b/wizard-work/beam/test_zeller_dow.py
@@ -0,0 +1,31 @@
+import unittest
+from zeller_dow import dow
+
+class TestZellerDow(unittest.TestCase):
+    def test_gap_dates_raise(self):
+        for day in range(5, 15):
+            with self.assertRaises(ValueError):
+                dow(1582, 10, day)
+
+    def test_silent_month_length_aliases_raise(self):
+        cases = [(2026, 2, 31), (2026, 2, 30), (2026, 4, 31),
+                 (1900, 2, 29), (2000, 2, 30), (2024, 13, 1),
+                 (2024, 0, 1), (2024, 1, 0), (2024, 1, 32)]
+        for y, m, d in cases:
+            with self.assertRaises(ValueError):
+                dow(y, m, d)
+
+    def test_valid_anchors(self):
+        # 1582-10-15 was a Friday (Gregorian cutover)
+        self.assertEqual(dow(1582, 10, 15), "Friday")
+        # 1582-10-04 Julian Thursday
+        self.assertEqual(dow(1582, 10, 4), "Thursday")
+        self.assertEqual(dow(2024, 2, 29), "Thursday")  # leap day exists
+        self.assertEqual(dow(2026, 10, 27), "Tuesday")
+
+    def test_leap_day_invalid_non_leap(self):
+        with self.assertRaises(ValueError):
+            dow(2023, 2, 29)
+
+if __name__ == "__main__":
+    unittest.main()
diff --git a/wizard-work/beam/zeller_dow.py b/wizard-work/beam/zeller_dow.py
index 98e908a..67a6a87 100644
--- a/wizard-work/beam/zeller_dow.py
+++ b/wizard-work/beam/zeller_dow.py
@@ -1,8 +1,26 @@
 """Day-of-week via Julian Day Number, handling Julian/Gregorian transition.
 
 Dates before 1582-10-15 use the Julian calendar; on/after use Gregorian.
-The gap 1582-10-05..14 did not exist.
+The gap 1582-10-05..14 did not exist, so those dates are invalid.
+Month lengths (including leap-year February) are validated per calendar.
 """
+def _julian_leap(y):
+    return y % 4 == 0
+
+def _gregorian_leap(y):
+    return (y % 4 == 0 and y % 100 != 0) or (y % 400 == 0)
+
+def _is_julian(y, m, d):
+    return (y, m, d) < (1582, 10, 15)
+
+def _days_in_month(y, m, julian):
+    if m in (1, 3, 5, 7, 8, 10, 12):
+        return 31
+    if m in (4, 6, 9, 11):
+        return 30
+    leap = _julian_leap(y) if julian else _gregorian_leap(y)
+    return 29 if leap else 28
+
 def _jdn_julian(y, m, d):
     a = (14 - m) // 12
     yy = y + 4800 - a
@@ -18,9 +36,15 @@ def _jdn_gregorian(y, m, d):
 NAMES = ["Monday", "Tuesday", "Wednesday", "Thursday", "Friday", "Saturday", "Sunday"]
 
 def dow(y, m, d):
-    if not (1 <= m <= 12 and 1 <= d <= 31):
-        raise ValueError("invalid month/day")
-    if (y, m, d) < (1582, 10, 15):
+    if not (1 <= m <= 12):
+        raise ValueError("invalid month")
+    julian = _is_julian(y, m, d)
+    # gap dates did not exist
+    if (1582, 10, 5) <= (y, m, d) <= (1582, 10, 14):
+        raise ValueError("date did not exist (Julian/Gregorian gap)")
+    if not (1 <= d <= _days_in_month(y, m, julian)):
+        raise ValueError("invalid day for month")
+    if julian:
         jdn = _jdn_julian(y, m, d)
     else:
         jdn = _jdn_gregorian(y, m, d)