None.

An integer, between 0 and 6, representing the day of the week for the given date according to local time: 0 for Sunday, 1 for Monday, 2 for Tuesday, and so on. Returns NaN if the date is invalid.

The return value of getDay() is zero-based, which is useful for indexing into arrays of days, for example:

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

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

However, for the purpose of internationalization, you should prefer using Intl.DateTimeFormat with the options parameter instead.

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"

The weekday variable has value 1, based on the value of the Date object xmas95, because December 25, 1995 is a Monday.

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

console.log(weekday); // 1

• Date.prototype.getUTCDate()
• Date.prototype.getUTCDay()
• Date.prototype.setDate()