How to change background image using javaScript with if conditions?

I am making a calendar clock and I want to change the background image per month using if statements. I am trying but all my ways are not working. Can you help me with this?

var tday = ["Sunday","Monday","Tuesday","Wednesday","Thursday","Friday","Saturday"];
    
var tmonth =["January","February","March","April","May","June","July","August","September","October","November","December"];
    
function getCalendar(){
    
    var d = new Date();
    var nday = d.getDay();
    var nmonth = d.getMonth();
    var ndate = d.getDate();
    var nyear = d.getFullYear();
    var dateText =""+tday[nday];
    var monthText = ""+tmonth[nmonth];
    var nCalendar = ""+monthText + " " + ndate + ", " + nyear
        
    document.getElementById('nDay').innerHTML = dateText;
    document.getElementById('nCalendar').innerHTML = nCalendar;
    document.getElementById("whatDay").style.backgroundImage = "url('images/january.png')";
}

<div id="whatDay">
    <div id="nDay" class="calendarDay">wednesday</div>
    <div id="nCalendar" class="calendar">December 30, 2050</div>
    <div id="clockbox" class="myClock"></div>
</div>