diff --git a/Userland/Libraries/LibJS/Runtime/ErrorTypes.h b/Userland/Libraries/LibJS/Runtime/ErrorTypes.h
index fa98a1e02b..3debebd25e 100644
--- a/Userland/Libraries/LibJS/Runtime/ErrorTypes.h
+++ b/Userland/Libraries/LibJS/Runtime/ErrorTypes.h
@@ -166,6 +166,7 @@
     M(StringNonGlobalRegExp, "RegExp argument is non-global")                                                                           \
     M(StringRawCannotConvert, "Cannot convert property 'raw' to object from {}")                                                        \
     M(StringRepeatCountMustBe, "repeat count must be a {} number")                                                                      \
+    M(TemporalAmbiguousMonthOfPlainMonthDay, "Accessing month of PlainMonthDay is ambiguous, use monthCode instead")                    \
     M(TemporalInvalidCalendar, "Invalid calendar")                                                                                      \
     M(TemporalInvalidCalendarFunctionResult, "Invalid calendar, {}() function returned undefined")                                      \
     M(TemporalInvalidCalendarIdentifier, "Invalid calendar identifier '{}'")                                                            \
diff --git a/Userland/Libraries/LibJS/Runtime/Temporal/CalendarPrototype.cpp b/Userland/Libraries/LibJS/Runtime/Temporal/CalendarPrototype.cpp
index bc76175f7b..ac01702ae7 100644
--- a/Userland/Libraries/LibJS/Runtime/Temporal/CalendarPrototype.cpp
+++ b/Userland/Libraries/LibJS/Runtime/Temporal/CalendarPrototype.cpp
@@ -10,6 +10,7 @@
 #include <LibJS/Runtime/Temporal/Calendar.h>
 #include <LibJS/Runtime/Temporal/CalendarPrototype.h>
 #include <LibJS/Runtime/Temporal/PlainDate.h>
+#include <LibJS/Runtime/Temporal/PlainMonthDay.h>
 #include <LibJS/Runtime/Temporal/PlainYearMonth.h>
 
 namespace JS::Temporal {
@@ -145,11 +146,15 @@ JS_DEFINE_NATIVE_FUNCTION(CalendarPrototype::month)
     // 3. Assert: calendar.[[Identifier]] is "iso8601".
     VERIFY(calendar->identifier() == "iso8601"sv);
 
-    // 4. If Type(temporalDateLike) is Object and temporalDateLike has an [[InitializedTemporalMonthDay]] internal slot, then
-    // a. Throw a TypeError exception.
-    // TODO
-
     auto temporal_date_like = vm.argument(0);
+
+    // 4. If Type(temporalDateLike) is Object and temporalDateLike has an [[InitializedTemporalMonthDay]] internal slot, then
+    if (temporal_date_like.is_object() && is<PlainMonthDay>(temporal_date_like.as_object())) {
+        // a. Throw a TypeError exception.
+        vm.throw_exception<TypeError>(global_object, ErrorType::TemporalAmbiguousMonthOfPlainMonthDay);
+        return {};
+    }
+
     // 5. If Type(temporalDateLike) is not Object or temporalDateLike does not have an [[InitializedTemporalDate]] or [[InitializedTemporalYearMonth]] internal slot, then
     if (!temporal_date_like.is_object() || !(is<PlainDate>(temporal_date_like.as_object()) || is<PlainYearMonth>(temporal_date_like.as_object()))) {
         // a. Set temporalDateLike to ? ToTemporalDate(temporalDateLike).
@@ -177,8 +182,7 @@ JS_DEFINE_NATIVE_FUNCTION(CalendarPrototype::month_code)
 
     auto temporal_date_like = vm.argument(0);
     // 4. If Type(temporalDateLike) is not Object or temporalDateLike does not have an [[InitializedTemporalDate]], [[InitializedTemporalMonthDay]], or [[InitializedTemporalYearMonth]] internal slot, then
-    // TODO PlainMonthDay objects
-    if (!temporal_date_like.is_object() || !(is<PlainDate>(temporal_date_like.as_object()) || is<PlainYearMonth>(temporal_date_like.as_object()))) {
+    if (!temporal_date_like.is_object() || !(is<PlainDate>(temporal_date_like.as_object()) || is<PlainMonthDay>(temporal_date_like.as_object()) || is<PlainYearMonth>(temporal_date_like.as_object()))) {
         // a. Set temporalDateLike to ? ToTemporalDate(temporalDateLike).
         temporal_date_like = to_temporal_date(global_object, temporal_date_like);
         if (vm.exception())
@@ -204,8 +208,7 @@ JS_DEFINE_NATIVE_FUNCTION(CalendarPrototype::day)
 
     auto temporal_date_like = vm.argument(0);
     // 4. If Type(temporalDateLike) is not Object or temporalDateLike does not have an [[InitializedTemporalDate]] or [[InitializedTemporalMonthDay]] internal slot, then
-    // TODO PlainMonthDay objects
-    if (!temporal_date_like.is_object() || !is<PlainDate>(temporal_date_like.as_object())) {
+    if (!temporal_date_like.is_object() || !(is<PlainDate>(temporal_date_like.as_object()) || is<PlainMonthDay>(temporal_date_like.as_object()))) {
         // a. Set temporalDateLike to ? ToTemporalDate(temporalDateLike).
         temporal_date_like = to_temporal_date(global_object, temporal_date_like);
         if (vm.exception())
diff --git a/Userland/Libraries/LibJS/Tests/builtins/Temporal/Calendar/Calendar.prototype.month.js b/Userland/Libraries/LibJS/Tests/builtins/Temporal/Calendar/Calendar.prototype.month.js
index 66b4045335..92fd756199 100644
--- a/Userland/Libraries/LibJS/Tests/builtins/Temporal/Calendar/Calendar.prototype.month.js
+++ b/Userland/Libraries/LibJS/Tests/builtins/Temporal/Calendar/Calendar.prototype.month.js
@@ -9,3 +9,16 @@ describe("correct behavior", () => {
         expect(calendar.month(date)).toBe(7);
     });
 });
+
+describe("errors", () => {
+    test("argument must not be a Temporal.PlainMonthDay object", () => {
+        const calendar = new Temporal.Calendar("iso8601");
+        const plainMonthDay = new Temporal.PlainMonthDay(7, 6);
+        expect(() => {
+            calendar.month(plainMonthDay);
+        }).toThrowWithMessage(
+            TypeError,
+            "Accessing month of PlainMonthDay is ambiguous, use monthCode instead"
+        );
+    });
+});