The 'validator' function of calendar will be available since next version, which allows the user determines what dates are selectable. Try download the preview plugin from
http://www.jeasyui.com/easyui/plugins/jquery.calendar.js and include to the page. The code below shows how to allow the user only select Mondays.
var c = $('#dd').datebox().datebox('calendar');
c.calendar('options').validator = function(date){
if (date.getDay() == 1){
return true;
}
}