title: First day of week (beginning with Sunday)
body: <p>I found that there is a function <code>last_day</code> for last day of month, and <code>date_part(dow, date)</code> for numeric day of week starting with Sunday, but I am trying to take a date, and get the first day of that week. </p>
  
  <p>Meaning: if date='<code>2018-02-14</code>' then result should be '<code>2018-02-11</code>'.</p>
  
  <p>Any ideas?</p>