makeSameDayArr.js

function makeSameDayArr(targetDay, firstDate){
  var targetDates = [],
      targetDate = new Date(firstDate.getFullYear(), 
                           firstDate.getMonth(), 
                           1.0 + (7-(firstDate.getDay()-targetDay)%7)%7);
  
  for(;targetDate.getMonth() == firstDate.getMonth();targetDate.setDate(targetDate.getDate()+7)){
    targetDates.push(new Date(targetDate));
  }
  return targetDates;
}

var days = {  'sun': 0,  'mon': 1,  'tue': 2,  'wed': 3,  'thu': 4,  'fri':5,  'sat':6 };
console.log(makeSameDayArr(days['wed'], new Date(2015, 7-1, 1)));

// [ Wed Jul 01 2015 00:00:00 GMT+0900 (東京 (標準時)),
//   Wed Jul 08 2015 00:00:00 GMT+0900 (東京 (標準時)),
//   Wed Jul 15 2015 00:00:00 GMT+0900 (東京 (標準時)),
//   Wed Jul 22 2015 00:00:00 GMT+0900 (東京 (標準時)),
//   Wed Jul 29 2015 00:00:00 GMT+0900 (東京 (標準時)) ]