DateJS.

today = new Date()
dayIndex = today.getDay()

0 = Sunday
1 = Monday
2 = Tuesday
3 = Wednesday
4 = Thursday
5 = Friday
6 = Saturday

и вы получите позицию того же дня этого месяца. то есть 15/02/2018 - третий четверг.

 /*return day postion and dayname */
    function weekAndDay(date) {
        var arrayWeek =[];
        var date = new Date(date),
        days = ['Sunday','Monday','Tuesday','Wednesday','Thursday','Friday','Saturday'],
        prefixes = ['','First', 'Second', 'Third', 'Fourth', 'Fifth'];
        if (date.getDate() % 7 == 0) {
            var prefixes2 = date.getDate() / 7;
        }else{
            var prefixes2 = Math.ceil(date.getDate() / 7);
        }
        arrayWeek.push({
            'dayPos' : prefixes[prefixes2] ,
            'dayName' : days[date.getDay()]
        });
        return arrayWeek;
    }

сделайте это просто ...

["Sunday","Monday","Tuesday","Wednesday","Thursday","Friday","Saturday"][(new Date()).getDay()]

Я не думал, что это будет так важно, но у меня есть 3 голоса за интернационализацию в комментариях ...

// dictionary version
({
  es: ["Domingo", "Lunes",  "Martes",  "Miércoles", "Jueves",   "Viernes", "Sábado"],
  en: ["Sunday",  "Monday", "Tuesday", "Wednesday", "Thursday", "Friday",  "Saturday"]
})['en'][(new Date()).getDay()]
// returns Wednesday

// list version
langs=['en','es']

[
  ["Domingo", "Lunes",  "Martes",  "Miércoles", "Jueves",   "Viernes", "Sábado"],
  ["Sunday",  "Monday", "Tuesday", "Wednesday", "Thursday", "Friday",  "Saturday"]
][langs.indexOf('es')][(new Date()).getDay()]
// returns Miércoles

И, наконец, код версии для гольфа ...

["Sun","Mon","Tues","Wednes","Thurs","Fri","Satur"][(new Date()).getDay()]+"day"

var days = [
    'SUN', //Sunday starts at 0
    'MON',
    'TUE',
    'WED',
    'THU',
    'FRI',
    'SAT'
];

d = new Date(); //This returns Wed Apr 02 2014 17:28:55 GMT+0800 (Malay Peninsula Standard Time)
x = d.getDay(); //This returns a number, starting with 0 for Sunday

alert (days[x]);

За работойиграть на скрипке.