index.js

var DoomsdayButton = document.getElementById("DoomsdayButton");
DoomsdayButton.addEventListener("click", function() {
    calculateDoomsday();
});

var YearNum = document.getElementById("YearNum");
var MonthNum = document.getElementById("MonthNum");
var DateNum = document.getElementById("DateNum");

var DoomsdayResult = document.getElementById("DoomsdayResult");

//Doomsday 0000 = monday (adjusted for leap year)
var Doomsday0000 = 1;

function calculateDoomsday() {

    //Clear output message
    DoomsdayResult.innerText = "";

    //Get input values
    var YearNumValue = YearNum.value;
    var MonthNumValue = MonthNum.value;
    var DateNumValue = DateNum.value;

    //Check valid inputs
    var doEnd = false;
    if(YearNumValue < 1) {
        DoomsdayResult.innerText += "Invalid year.\n";
        doEnd = true;
    }
    if(MonthNumValue < 1 || MonthNumValue > 12) {
        DoomsdayResult.innerText += "Invalid month.\n";
        doEnd = true;
    }
    if(DateNumValue < 1 || DateNumValue > 31) {
        DoomsdayResult.innerText += "Invalid date.\n";
        doEnd = true;        
    }
    //If error, return
    if(doEnd) {
        return;
    }

    //Anchor calculation
    var YearCentury = Math.floor(YearNumValue/100);
    var isLeapCentury = YearCentury % 4 == 0;

    //Calculate simple anchor doomsday
    var AnchorDoomsday = Math.abs(Doomsday0000 + (YearCentury * 2) + 2);
    if(isLeapCentury) { //If anchor century was a leap year, doomsday is 29, so +=1
        AnchorDoomsday += 1;
    }
    AnchorDoomsday = AnchorDoomsday % 7;

    //Year calculation
    var YearSinceCentury = YearNumValue - (YearCentury*100);
    var isLeapYear = YearSinceCentury % 4 == 0;
    var YearDoomsday = AnchorDoomsday + YearSinceCentury + Math.floor(YearSinceCentury / 4);

    //Month calulation
    var MonthDoomsday;
    switch(MonthNumValue) {
        case "1": //January
            if(isLeapYear) {
                MonthDoomsday = 32;
            } else {
                MonthDoomsday = 31;
            }
            break;
        case "2": //February
            if(isLeapYear) {
                MonthDoomsday = 29;
            } else {
                MonthDoomsday = 28;
            }
            break;
        case"3": //March
            MonthDoomsday = 7;
            break;
        case "4": //April
            MonthDoomsday = 4;
            break;
        case "5": //May
            MonthDoomsday = 9;
            break;
        case "6": //June
            MonthDoomsday = 6;
            break;
        case "7": //July
            MonthDoomsday = 11;
            break;
        case "8": //August
            MonthDoomsday = 8;
            break;
        case "9": //Septempber
            MonthDoomsday = 5;
            break;
        case "10": //October
            MonthDoomsday = 10;
            break;
        case "11": //November
            MonthDoomsday = 7;
            break;
        case "12": //December
            MonthDoomsday = 12;
            break;
        default:
            alert("Incorrect month: " + MonthNumValue);
            break;
    }

    //Date calculation
    var DateDifference = DateNumValue - MonthDoomsday;
    var DateDoomsday = YearDoomsday + DateDifference;
    while(DateDoomsday < 0) {
        DateDoomsday += 7;
    }
    DateDoomsday = DateDoomsday % 7;
    
    DoomsdayResult.innerText = DateNumValue + "/" + MonthNumValue + "/" + YearNumValue + " is a:";

    //Output result
    switch(DateDoomsday) {
        case 0:
            DoomsdayResult.innerText += " Sunday";
            break;
        case 1:
            DoomsdayResult.innerText += " Monday";
            break;
        case 2:
            DoomsdayResult.innerText += " Tuesday";
            break;
        case 3:
            DoomsdayResult.innerText += " Wednesday";
            break;
        case 4:
            DoomsdayResult.innerText += " Thursday";
            break;
        case 5:
            DoomsdayResult.innerText += " Friday";
            break;
        case 6:
            DoomsdayResult.innerText += " Saturday";
            break;
        default:
            alert("Incorrect day of week");
    }
}