In this last week I have developed an application that uses ASP.NET and some AJAX Toolkit Control ver. 1.0.11119.0.
I noticed that when the CalendarExtender was inside an UpdatePanel it lost the css style information.
To avoid this problem I found this thread where you can read that you need to specify the css style in your page or in your css file, in this way:
.ajax__calendar_container {padding:4px;position:absolute;cursor:default;width:170px;font-size:11px;text-align:center;font-family:tahoma,verdana,helvetica;}
.ajax__calendar_body {height:139px;width:170px;position:relative;overflow:hidden;margin:auto;}
.ajax__calendar_days, .ajax__calendar_months, .ajax__calendar_years {top:0px;left:0px;height:139px;width:170px;position:absolute;text-align:center;margin:auto;}
.ajax__calendar_container TABLE {font-size:11px;}
.ajax__calendar_header {height:20px;width:100%;}
.ajax__calendar_prev {cursor:pointer;width:15px;height:15px;float:left;background-repeat:no-repeat;background-position:50% 50%;
background-image:url(WebResource.axd?d=x6lt9HfG5Si8NZjMNt1DLcY6US8hUl3G4muzIFtahMQFZTbwxJ00qsx5N8jeAW90ZvGxZ6SJJh48yx62JKItzQ2&t=633053122925312500);}
.ajax__calendar_next {cursor:pointer;width:15px;height:15px;float:right;background-repeat:no-repeat;background-position:50% 50%;
background-image:url(WebResource.axd?d=x6lt9HfG5Si8NZjMNt1DLcY6US8hUl3G4muzIFtahMQFZTbwxJ00qsx5N8jeAW90HZemRms4wXOo1BZWTwWtNQ2&t=633053122925312500);}
.ajax__calendar_title {cursor:pointer;font-weight:bold;}
.ajax__calendar_footer {height:15px;}
.ajax__calendar_today {cursor:pointer;padding-top:3px;}
.ajax__calendar_dayname {height:17px;width:17px;text-align:right;padding:0 2px;}
.ajax__calendar_day {height:17px;width:18px;text-align:right;padding:0 2px;cursor:pointer;}
.ajax__calendar_month {height:44px;width:40px;text-align:center;cursor:pointer;overflow:hidden;}
.ajax__calendar_year {height:44px;width:40px;text-align:center;cursor:pointer;overflow:hidden;}
.ajax__calendar .ajax__calendar_container {border:1px solid #646464;background-color:#ffffff;color:#000000;}
.ajax__calendar .ajax__calendar_footer {border-top:1px solid #f5f5f5;}
.ajax__calendar .ajax__calendar_dayname {border-bottom:1px solid #f5f5f5;}
.ajax__calendar .ajax__calendar_day {border:1px solid #ffffff;}
.ajax__calendar .ajax__calendar_month {border:1px solid #ffffff;}
.ajax__calendar .ajax__calendar_year {border:1px solid #ffffff;}
.ajax__calendar .ajax__calendar_active .ajax__calendar_day {background-color:#edf9ff;border-color:#0066cc;color:#0066cc;}
.ajax__calendar .ajax__calendar_active .ajax__calendar_month {background-color:#edf9ff;border-color:#0066cc;color:#0066cc;}
.ajax__calendar .ajax__calendar_active .ajax__calendar_year {background-color:#edf9ff;border-color:#0066cc;color:#0066cc;}
.ajax__calendar .ajax__calendar_other .ajax__calendar_day {background-color:#ffffff;border-color:#ffffff;color:#646464;}
.ajax__calendar .ajax__calendar_other .ajax__calendar_year {background-color:#ffffff;border-color:#ffffff;color:#646464;}
.ajax__calendar .ajax__calendar_hover .ajax__calendar_day {background-color:#edf9ff;border-color:#daf2fc;color:#0066cc;}
.ajax__calendar .ajax__calendar_hover .ajax__calendar_month {background-color:#edf9ff;border-color:#daf2fc;color:#0066cc;}
.ajax__calendar .ajax__calendar_hover .ajax__calendar_year {background-color:#edf9ff;border-color:#daf2fc;color:#0066cc;}
.ajax__calendar .ajax__calendar_hover .ajax__calendar_title {color:#0066cc;}
.ajax__calendar .ajax__calendar_hover .ajax__calendar_today {color:#0066cc;}