diff --git a/Userland/Libraries/LibJS/Runtime/Temporal/Calendar.cpp b/Userland/Libraries/LibJS/Runtime/Temporal/Calendar.cpp
index 301a933b10..6e64673d0c 100644
--- a/Userland/Libraries/LibJS/Runtime/Temporal/Calendar.cpp
+++ b/Userland/Libraries/LibJS/Runtime/Temporal/Calendar.cpp
@@ -525,4 +525,19 @@ String iso_month_code(Object& temporal_object)
     VERIFY_NOT_REACHED();
 }
 
+// 12.1.44 ISODay ( temporalObject ), https://tc39.es/proposal-temporal/#sec-temporal-isomonthcode
+u8 iso_day(Object& temporal_object)
+{
+    // 1. Assert: temporalObject has an [[ISODay]] internal slot.
+    // NOTE: Asserted by the VERIFY_NOT_REACHED at the end
+
+    // 2. Return 𝔽(temporalObject.[[ISODay]]).
+    // TODO: add the rest of the builtins with a [[ISODay]] slot (PlainYearMonth, PlainMonthDay)
+    if (is<PlainDate>(temporal_object))
+        return static_cast<PlainDate&>(temporal_object).iso_day();
+    if (is<PlainDateTime>(temporal_object))
+        return static_cast<PlainDateTime&>(temporal_object).iso_day();
+    VERIFY_NOT_REACHED();
+}
+
 }
diff --git a/Userland/Libraries/LibJS/Runtime/Temporal/Calendar.h b/Userland/Libraries/LibJS/Runtime/Temporal/Calendar.h
index 4c46fc8e15..95f0cd37cf 100644
--- a/Userland/Libraries/LibJS/Runtime/Temporal/Calendar.h
+++ b/Userland/Libraries/LibJS/Runtime/Temporal/Calendar.h
@@ -50,5 +50,6 @@ Optional<TemporalDate> iso_date_from_fields(GlobalObject&, Object& fields, Objec
 i32 iso_year(Object& temporal_object);
 u8 iso_month(Object& temporal_object);
 String iso_month_code(Object& temporal_object);
+u8 iso_day(Object& temporal_object);
 
 }
diff --git a/Userland/Libraries/LibJS/Runtime/Temporal/CalendarPrototype.cpp b/Userland/Libraries/LibJS/Runtime/Temporal/CalendarPrototype.cpp
index a49f958ab4..0dddef4d0e 100644
--- a/Userland/Libraries/LibJS/Runtime/Temporal/CalendarPrototype.cpp
+++ b/Userland/Libraries/LibJS/Runtime/Temporal/CalendarPrototype.cpp
@@ -34,6 +34,7 @@ void CalendarPrototype::initialize(GlobalObject& global_object)
     define_native_function(vm.names.year, year, 1, attr);
     define_native_function(vm.names.month, month, 1, attr);
     define_native_function(vm.names.monthCode, month_code, 1, attr);
+    define_native_function(vm.names.day, day, 1, attr);
     define_native_function(vm.names.toString, to_string, 0, attr);
     define_native_function(vm.names.toJSON, to_json, 0, attr);
 }
@@ -180,6 +181,33 @@ JS_DEFINE_NATIVE_FUNCTION(CalendarPrototype::month_code)
     return js_string(vm, iso_month_code(temporal_date_like.as_object()));
 }
 
+// 12.4.12 Temporal.Calendar.prototype.day ( temporalDateLike ), https://tc39.es/proposal-temporal/#sec-temporal.calendar.prototype.day
+// NOTE: This is the minimum day implementation for engines without ECMA-402.
+JS_DEFINE_NATIVE_FUNCTION(CalendarPrototype::day)
+{
+    // 1. Let calendar be the this value.
+    // 2. Perform ? RequireInternalSlot(calendar, [[InitializedTemporalCalendar]]).
+    auto* calendar = typed_this(global_object);
+    if (vm.exception())
+        return {};
+
+    // 3. Assert: calendar.[[Identifier]] is "iso8601".
+    VERIFY(calendar->identifier() == "iso8601"sv);
+
+    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())) {
+        // a. Set temporalDateLike to ? ToTemporalDate(temporalDateLike).
+        temporal_date_like = to_temporal_date(global_object, temporal_date_like);
+        if (vm.exception())
+            return {};
+    }
+
+    // 5. Return ! ISODay(temporalDateLike).
+    return Value(iso_day(temporal_date_like.as_object()));
+}
+
 // 12.4.23 Temporal.Calendar.prototype.toString ( ), https://tc39.es/proposal-temporal/#sec-temporal.calendar.prototype.tostring
 JS_DEFINE_NATIVE_FUNCTION(CalendarPrototype::to_string)
 {
diff --git a/Userland/Libraries/LibJS/Runtime/Temporal/CalendarPrototype.h b/Userland/Libraries/LibJS/Runtime/Temporal/CalendarPrototype.h
index b5168edc13..3d2a5b0e92 100644
--- a/Userland/Libraries/LibJS/Runtime/Temporal/CalendarPrototype.h
+++ b/Userland/Libraries/LibJS/Runtime/Temporal/CalendarPrototype.h
@@ -24,6 +24,7 @@ private:
     JS_DECLARE_NATIVE_FUNCTION(year);
     JS_DECLARE_NATIVE_FUNCTION(month);
     JS_DECLARE_NATIVE_FUNCTION(month_code);
+    JS_DECLARE_NATIVE_FUNCTION(day);
     JS_DECLARE_NATIVE_FUNCTION(to_string);
     JS_DECLARE_NATIVE_FUNCTION(to_json);
 };
diff --git a/Userland/Libraries/LibJS/Tests/builtins/Temporal/Calendar/Calendar.prototype.day.js b/Userland/Libraries/LibJS/Tests/builtins/Temporal/Calendar/Calendar.prototype.day.js
new file mode 100644
index 0000000000..421dd40cee
--- /dev/null
+++ b/Userland/Libraries/LibJS/Tests/builtins/Temporal/Calendar/Calendar.prototype.day.js
@@ -0,0 +1,11 @@
+describe("correct behavior", () => {
+    test("length is 1", () => {
+        expect(Temporal.Calendar.prototype.day).toHaveLength(1);
+    });
+
+    test("basic functionality", () => {
+        const calendar = new Temporal.Calendar("iso8601");
+        const date = new Temporal.PlainDate(2021, 7, 23);
+        expect(calendar.day(date)).toBe(23);
+    });
+});