Edit in JSFiddle

$(function() {
    var $toggle = $('.christmas-toggle'),
        $body = $('body'),
        $window = $(window),
        speed = 0.1,
        horizontalVariance = 100,
        createInterval = 100,
        doChristmas = function() {
            var wndHeight = $window.height(), // Get wind. dimensions often since the user may resize the browser.
                wndWidth = $window.width(), 
                vertSpeed = speed * wndHeight,
                startX = Math.random() * wndWidth,
                endX = startX + ((Math.random() - 0.5) * horizontalVariance),
                $snowFlake = $('<div>', {
                    text: "*",
                    addClass: "snow-flake"
                });
            if (!$toggle[0].checked) {
                return;
            }
            $body.prepend($snowFlake);
            $snowFlake.css({
                left: startX
            }).animate({
                left: endX,
                top: wndHeight,
                opacity: 0
            }, wndHeight / speed, 'linear', function() {
                $snowFlake.remove();
            });
            setTimeout(doChristmas, createInterval);
        };
    $toggle.click(doChristmas);
    doChristmas();
});

<label>
    <input type="checkbox" class="christmas-toggle" name="christmas" checked />
    christmas
</label>

.snow-flake {
position: fixed;
top: -10;
}