I once had a problem with this extender back then. I've searched for the answer long and hard and never found it. The search results showed me that I was better off using the ASP.NET calendar with a PopupExtender, because there was no way that the SelectedDate in the CalendarExtender could be retrieved by the server, it stays on the client side.

 
The regex for email addresses in the MSDN website sucks a bit, it may work in code-behind but not in the .aspx file in which the server script for RegularExpressionValidator is found. Insert this string into the ValidationExpression property of your RegularExpressionValidator: (works like a charm)

"^[a-zA-Z0-9]+([\._]?[a-zA-Z0-9]+)*@[a-zA-Z0-9]+([\.-]?[a-zA-Z]+)?(\.[a-zA-Z]{2,3})+$"

I found this piece of code in this site: (it is found near the bottom of the page)
Thanks so much "djrulz"
 
I was working on a website and found myself desiring to have the CreateUserWizard inside a Modal Popup Extender. But because the CreateUserWizard had to have a full postback in order for it to be able to pass the parameters back to the server, a workaround is necessary. I had to search for a solution long & hard, at last I found a solution. This works like a charm: (click the link below)
All thanks to the author of this site

By the way, it's in VB, but not to worry for C# coders. There are a lot of websites offering VB to C# conversion, any search engine would do the trick.