The Calendar webpart's display is a little limiting, you may be better off using Javascript to wrap the elements manually, something like this:
$(".calendar-item").each(function() {
var parent = $($(this).parent());
if(parent.is("td")) {
var newContainer = $('<div class="calendar-cell-inner"></div>');
$(".calendar-item", parent).appendTo(newContainer);
// may need this
// parent.html("");
parent.append(newContainer);
}
});
Totally untested, but honestly may be the easiest way to go. Otherwise you need to create your own Repeater, which can be done using windowing functions (to detect when a new day is).