Quickly Find Day of the Week – C#

Hi all!

This is just a random post to show off something new I found. I don’t claim credit for this algorithm and if you’re curious as to how it works then this Wikipedia article may be of interest to you. This post has a good explanation of why the algorithm works in the first place.

Given a date the following function would give a string donating the weekday on which a given date will fall:

public string FastDayOfTheWeek(int year, int month, int day)
{
    string[] days = new string[] { "Sunday", "Monday", "Tuesday", "Wednesday", "Thursday", "Friday", "Saturday" };

    if (month < 3)
    {
        --year;
    }

    int weekday = (year + (year / 4) - (year / 100) + (year / 400) + (int)("032504514624"[month - 1] - 48) + day) % 7;

    return days[weekday];
}

If you run the function for the 1st of March 2020 it will give a Sunday. This is the correct answer. This function works backwards and forwards in time as far as I can tell.

Another interesting thing about the given formula is that it could easily be memorised and may make for a nice mathematical party trick.

Hopefully someone out there will find this useful and or interesting.