let bodyStyle = document.body.style;

switch (new Date().getMonth()) {
    case 10:
        bodyStyle.backgroundImage = "url('/images/halloween_background_1.gif')";
    case 11:
        bodyStyle.backgroundImage = "url('/images/jwsfp1.gif')";
    case 12:
        bodyStyle.backgroundImage = "url('/images/christmas.gif')";
}