title: How to change the day that Oracle TRUNC uses to start the week
body: <p>I have the following SQL:</p>
  
  <pre><code>SELECT SUM(theNumberCol) qty, TRUNC(theDateCol, 'WW') weekDate
  FROM theTable
  GROUP BY TRUNC(theDateCol, 'WW');
  </code></pre>
  
  <p>This works fine for finding the sum of theNumberCol for each week. The problem is that the week seems to start on a Tuesday. For example "12/17/2013 8:56:05 AM" is truncated to "12/17/2013" while "12/16/2013 5:09:25 AM" is truncated to "12/10/2013".</p>
  
  <ol>
  <li>How does Oracle determine which day to start the week?</li>
  <li>Can I change the week to start on Saturday?</li>
  </ol>