change css property each day
let element = document.querySelector('.change-my-colour');
let date = new Date();
switch(date.getDay()) { /* use date.getDate() instead for days of the month */
case 0: /* Sunday */
element.style.background = "red";
break;
case 1: /* Monday */
element.style.background = "orange";
break;
case 2: /* Tuesday */
element.style.background = "yellow";
break;
/* etc... */
default:
break;
}