// Set week day to Sunday, with the default weekStartsOn of Sunday:
var result = setDay(new Date(2014, 8, 1), 0)
//=> Sun Aug 31 2014 00:00:00
// Set week day to Sunday, with a weekStartsOn of Monday:
var result = setDay(new Date(2014, 8, 1), 0, { weekStartsOn: 1 })
//=> Sun Sep 07 2014 00:00:00