We cannot create schedules with a MonthlyInterval, looking at the code it forces the interval to be a tuple. 

tableau_auth = TSC.PersonalAccessTokenAuth(tableau_username, tableau_password)
server = TSC.Server(tableau_server, use_server_version=True)
with server.auth.sign_in(tableau_auth):
    schedule = TSC.ScheduleItem(
        name="my_test_schedule",
        priority=50,
        schedule_type=TSC.ScheduleItem.Type.Extract,
        execution_order=TSC.ScheduleItem.ExecutionOrder.Parallel,
        interval_item=TSC.MonthlyInterval(time(5, 30), 5),
    )

    new_schedule = server.schedules.create(schedule)

Traceback (most recent call last):
  File "/MY_REPO/_develop_schedules.py", line 86, in <module>
    new_schedule = server.schedules.create(schedule)
                   ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/VENV_PYTHON_PATH/site-packages/tableauserverclient/server/endpoint/endpoint.py", line 274, in wrapper
    return func(self, *args, **kwargs)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/VENV_PYTHON_PATH/site-packages/tableauserverclient/server/endpoint/schedules_endpoint.py", line 80, in create
    server_response = self.post_request(url, create_req)
                      ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/VENV_PYTHON_PATH/site-packages/tableauserverclient/server/endpoint/endpoint.py", line 226, in post_request
    return self._make_request(
           ^^^^^^^^^^^^^^^^^^^
  File "/VENV_PYTHON_PATH/site-packages/tableauserverclient/server/endpoint/endpoint.py", line 141, in _make_request
    self._check_status(server_response, url)
  File "/VENV_PYTHON_PATH/site-packages/tableauserverclient/server/endpoint/endpoint.py", line 166, in _check_status
    raise ServerResponseError.from_response(server_response.content, self.parent_srv.namespace, url)
tableauserverclient.server.endpoint.exceptions.ServerResponseError: 

        409004: Bad Request
                Schedule monthDay must be either an integer between 1 and 31 or 'LastDay'.