title: Run Airflow Dag only once
body: <p>I am looking to create <code>@once</code> run for airflow but it doesn't get triggered. Here is my configuration.</p>
  <pre><code>default_args = {'owner': 'airflow',
                  'start_date': datetime(2020, 5, 1),
                  'email': ['your_email@somecompany.com'],
                  'email_on_failure': False,
                  'email_on_retry': False
                  }
  
  dag = DAG('airflow_spark_dag',
            default_args=default_args,
            schedule_interval='@once',
            catchup=False
            )
  </code></pre>
  <p>Even tried using this command <code>airflow unpause airflow_spark_dag</code></p>
  <p>I am using airflow v1.10.12.</p>
  <p>The list_dag_run is empty and there is no error. Changing schedule_interval to <code>@hourly</code> works but not <code>@once</code></p>