A test Airflow dag to show the monthly "schedule_interval".

hello.py

from airflow.operators import BashOperator
from airflow.models import DAG
from datetime import datetime, timedelta


args = {
    'start_date': datetime.utcnow(),
    'owner': 'airflow',
}

one_month_ago = datetime.combine(datetime.today() - timedelta(30),
                                  datetime.min.time())


dag = DAG(
    dag_id='hello',
    default_args=args,
    schedule_interval="@monthly")
    #schedule_interval=None)



bash_task = BashOperator(
    task_id="bash_task",
    bash_command='echo "hello world"',
    dag=dag)

To run this dag, you could use "backfill" to test it, for example, 4 runs from 2016-01-01 to 2016-04-01.
airflow backfill hello -s 2016-01-01 -e 2016-04-01