var currentDate = new Date();
var monat = currentDate.getMonth();

function datum (){

if (monat == 0) {
    document.body.style.backgroundImage = 'url("images/sitebg_winter.jpg")';
} else if (monat == 1) {
    document.body.style.backgroundImage = 'url("images/sitebg_winter.jpg")';
} else if (monat == 2) {
    document.body.style.backgroundImage = 'url("images/sitebg_test.jpg")';
} else if (monat == 3) {
    document.body.style.backgroundImage = 'url("images/sitebg_test.jpg")';
}
else if (monat == 4) {
    document.body.style.backgroundImage = 'url("images/sitebg_test.jpg")';
} else if (monat == 5) {
    document.body.style.backgroundImage = 'url("images/sitebg_sommer.jpg")';
} else if (monat == 6) {
    document.body.style.backgroundImage = 'url("images/sitebg_sommer.jpg")';
} else if (monat == 7) {
    document.body.style.backgroundImage = 'url("images/sitebg_sommer.jpg")';
}
if (monat == 8) {
    document.body.style.backgroundImage = 'url("images/sitebg_herbst.jpg")';
} else if (monat == 9) {
    document.body.style.backgroundImage = 'url("images/sitebg_herbst.jpg")';
} else if (monat == 10) {
    document.body.style.backgroundImage = 'url("images/sitebg_herbst.jpg")';
} else if (monat == 11) {
    document.body.style.backgroundImage = 'url("images/sitebg_winter.jpg")';
}
}