$(function () {
$("#Dateis").datepicker().hover(function () {
$(this).closest('td').addClass('active');
}, function () {
$(this).closest('td').removeClass('active');
});
});
Or
$(function () {
$("#Dateis").datepicker().on('mouseenter mouseleave',function(){
$(this).closest('td').toggleClass('active');
});
});
Update After OP's comment
$(document).on("mouseenter mouseleave", ".ui-state-default", function () {
$(this).closest('td').toggleClass('ui-state-active');
});
Remove today's date highlight
.ui-datepicker-today a.ui-state-highlight {
border-color: #d3d3d3;
background: #e6e6e6 url(/themeroller/images/ui-bg_glass_75_e6e6e6_1x400.png) 50% 50% repeat-x;
color: #555555;
}