The problem I was seeing was that the Calendar would always popup when the page loaded, even though that field didn't have focus. The problem ended up being simple, but I didn't see it documented anywhere: I had the code in the folowing order:
- ASP:Textbox
- AJAX:MaskedEditExtender
- AJAX:MaskedEditValidator
- AJAX: Calendar Extender
Moving the Calendar Extendar to the top of the extenders resolved the issue.
Thank you so much! I was stuck with this problem for the last 2 days.