I think it will solve your problem..
function eventWindow(url) {
event_popupWin = window.open(url, 'event', 'resizable=yes,scrollbars=yes,toolbar=no,width=400,height=400');
event_popupWin.opener = self;
}
<div class="day-number">
<a href="javascript:eventWindow('event.php?&d=<?php echo $list_day; ?>&m=<?php echo $month; ?>&y=<?php echo $year; ?>')"><?php echo $list_day; ?></a>
</div>