Date.prototype.getDay()

const birthday = new Date("August 19, 1975 23:15:30");
const day1 = birthday.getDay();
//  - 0 - 6

console.log(day1);
// 2

getDay()

const valentines = new Date("1995-02-14");
const day = valentines.getDay();
const dayNames = ["", "", "" /* ,  */];

console.log(dayNames[day]); // ""

const options = { weekday: "long" };
console.log(new Intl.DateTimeFormat("en-US", options).format(valentines));
// "Monday"
console.log(new Intl.DateTimeFormat("de-DE", options).format(valentines));
// "Montag"

const xmas95 = new Date("1995-12-25T23:15:30");
const weekday = xmas95.getDay();

console.log(weekday); // 1