title: Get the day of the last Friday of each month
body: <p>I am new with ruby and I want to get the day of the last Friday of each month.
  For example, the last Friday of March is 29, the Last Friday of April 26.
  So, how can I get a solution? 
  I'm using the rails framework.</p>
  
  <p>The method <code>.cweek</code> returns the week of the year, but does not return the week of the current month.</p>