// Set Sunday to 1 September 2014:
var result = setDay(new Date(2014, 8, 1), 0)
//=> Sun Aug 31 2014 00:00:00
// If week starts with Monday, set Sunday to 1 September 2014:
var result = setDay(new Date(2014, 8, 1), 0, { weekStartsOn: 1 })
//=> Sun Sep 07 2014 00:00:00