Finding day of week from date (day, month, year) in JavaScript

We are required to write a JavaScript function that takes in three arguments, namely: day, month and year. Based on these three inputs, our function should find the day of the week on that date.

For example: If the inputs are ?

day = 15,
month = 8,
year = 1993

Output

Then the output should be ?

const output = 'Sunday'

Using Built-in Date Object (Simple Method)

The easiest approach is to use JavaScript's built-in Date object:

function getDayOfWeek(day, month, year) {
    const date = new Date(year, month - 1, day); // month is 0-indexed
    const days = ['Sunday', 'Monday', 'Tuesday', 'Wednesday', 'Thursday', 'Friday', 'Saturday'];
    return days[date.getDay()];
}

console.log(getDayOfWeek(15, 8, 1993)); // Sunday
console.log(getDayOfWeek(1, 1, 2024));  // Monday

Sunday
Monday

Manual Calculation Method

For educational purposes, here's how to calculate the day manually by counting days from a reference point:

const dayOfTheWeek = (day, month, year) => {
    // JS months start at 0
    return dayOfTheWeekJS(day, month - 1, year);
}

function dayOfTheWeekJS(day, month, year) {
    const DAYS = [
        'Sunday',
        'Monday',
        'Tuesday',
        'Wednesday',
        'Thursday',
        'Friday',
        'Saturday',
    ];
    const DAY_1970_01_01 = 4; // January 1, 1970 was Thursday (index 4)
    let days = day - 1;
    
    // Add days from previous months
    while (month - 1 >= 0) {
        days += daysInMonthJS(month - 1, year);
        month -= 1;
    }
    
    // Add days from previous years
    while (year - 1 >= 1970) {
        days += daysInYear(year - 1);
        year -= 1;
    }
    
    return DAYS[(days + DAY_1970_01_01) % DAYS.length];
}

function daysInMonthJS(month, year) {
    const days = [
        31, // January
        28 + (isLeapYear(year) ? 1 : 0), // February
        31, // March
        30, // April
        31, // May
        30, // June
        31, // July
        31, // August
        30, // September
        31, // October
        30, // November
        31, // December
    ];
    return days[month];
}

function daysInYear(year) {
    return 365 + (isLeapYear(year) ? 1 : 0);
}

function isLeapYear(year) {
    return year % 4 === 0 && year % 100 !== 0 || year % 400 === 0;
}

console.log(dayOfTheWeek(15, 8, 1993));

Sunday

Explanation

The manual calculation method works by counting total days from Unix epoch (January 1, 1970, which was a Thursday). The algorithm:

• Count days in complete years since 1970

• Count days in complete months for the current year

• Add remaining days for the incomplete month

• Use modulo 7 to find the corresponding day of the week

Comparison

Conclusion

For practical applications, use JavaScript's built-in Date object with getDay(). The manual calculation approach is useful for understanding date algorithms and works without relying on built-in functions.