diff --git a/tableauserverclient/models/interval_item.py b/tableauserverclient/models/interval_item.py
index 14cec187..0888441f 100644
--- a/tableauserverclient/models/interval_item.py
+++ b/tableauserverclient/models/interval_item.py
@@ -305,6 +305,7 @@ def interval(self, interval_values):
             "Fourth",
             "Fifth",
             "Last",
+            "Customized Monthly",
         ]
         for value in range(1, 32):
             VALID_INTERVALS.append(str(value))
diff --git a/test/assets/schedule_get_customized_monthly_id.xml b/test/assets/schedule_get_customized_monthly_id.xml
new file mode 100644
index 00000000..cc2bf560
--- /dev/null
+++ b/test/assets/schedule_get_customized_monthly_id.xml
@@ -0,0 +1,10 @@
+<?xml version='1.0' encoding='UTF-8'?>
+<tsResponse xmlns="http://tableau.com/api" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:schemaLocation="http://tableau.com/api http://tableau.com/api/ts-api-2.3.xsd">
+  <schedule id="f048d794-90dc-40b0-bfad-2ca78e437369" name="Monthly customized" state="Active" priority="50" createdAt="2024-03-13T17:05:01Z" updatedAt="2024-03-16T16:05:14Z" type="Extract" frequency="Monthly" nextRunAt="2024-03-24T16:05:00Z" executionOrder="Parallel">
+    <frequencyDetails start="17:05:00">
+      <intervals>
+        <interval monthDay="Customized Monthly"/>
+      </intervals>
+    </frequencyDetails>
+  </schedule>
+</tsResponse>
diff --git a/test/test_schedule.py b/test/test_schedule.py
index 45e35ec2..823a8760 100644
--- a/test/test_schedule.py
+++ b/test/test_schedule.py
@@ -15,6 +15,7 @@
 GET_DAILY_ID_XML = TEST_ASSET_DIR / "schedule_get_daily_id.xml"
 GET_MONTHLY_ID_XML = TEST_ASSET_DIR / "schedule_get_monthly_id.xml"
 GET_MONTHLY_ID_2_XML = TEST_ASSET_DIR / "schedule_get_monthly_id_2.xml"
+GET_CUSTOMIZED_MONTHLY_ID_XML = TEST_ASSET_DIR / "schedule_get_customized_monthly_id.xml"
 GET_EMPTY_XML = TEST_ASSET_DIR / "schedule_get_empty.xml"
 CREATE_HOURLY_XML = TEST_ASSET_DIR / "schedule_create_hourly.xml"
 CREATE_DAILY_XML = TEST_ASSET_DIR / "schedule_create_daily.xml"
@@ -178,6 +179,21 @@ def test_get_monthly_by_id_2(server: TSC.Server) -> None:
         assert ("Monday", "First") == schedule.interval_item.interval
 
 
+def test_get_customized_monthly_by_id(server: TSC.Server) -> None:
+    server.version = "3.15"
+    response_xml = GET_CUSTOMIZED_MONTHLY_ID_XML.read_text()
+    with requests_mock.mock() as m:
+        schedule_id = "f048d794-90dc-40b0-bfad-2ca78e437369"
+        baseurl = f"{server.baseurl}/schedules/{schedule_id}"
+        m.get(baseurl, text=response_xml)
+        schedule = server.schedules.get_by_id(schedule_id)
+        assert schedule is not None
+        assert schedule_id == schedule.id
+        assert "Monthly customized" == schedule.name
+        assert "Active" == schedule.state
+        assert ("Customized Monthly",) == schedule.interval_item.interval
+
+
 def test_delete(server: TSC.Server) -> None:
     with requests_mock.mock() as m:
         m.delete(server.schedules.baseurl + "/c9cff7f9-309c-4361-99ff-d4ba8c9f5467", status_code=204)