Code.gs

var DAYSOFWEEK = [ScriptApp.WeekDay.SUNDAY, ScriptApp.WeekDay.MONDAY, ScriptApp.WeekDay.TUESDAY,
               ScriptApp.WeekDay.WEDNESDAY, ScriptApp.WeekDay.THURSDAY,                                            
               ScriptApp.WeekDay.FRIDAY, ScriptApp.WeekDay.SATURDAY];

String.prototype.capitalize = function() {
    return this.charAt(0).toUpperCase() + this.slice(1);
}

function isValidDate(d) {
  return d instanceof Date && !isNaN(d);
}

function SPLITWEEKDAYJOIN(input){
  var output = [];
  var days = input.split("\n");
  for(var day in days) {
    var date = new Date(days[day]);
    if (isValidDate(date)) {
      output.push(String(DAYSOFWEEK[date.getDay()]).toLowerCase().capitalize())
    }
  }
  return output.join("\n");
}