title: File dependency in Airflow
body: <p>I have a Airflow DAG and I want to trigger it on particular day of each month. It is a table migration task so I don't want to move it unless tables in directory have been updated. </p>
  
  <p>How can I do something like the DAG executes on that day only if there's some file in that directory? Or is there a better way to do it like check if files have been updated. </p>
  
  <p>Please suggest me some solution and the way to do it.</p>