See Guys,the problem may be because you have forgotten to set PopupPosition of CalendarExtender.Better specify the PopupPosition to "BottomLeft" and try.
If still the problem exists it may be because of the table you have taken.You might have placed the TextBox Control and CalenderExtender in a table.
Most often we are habituated to drop the table from Layout --> Insert Table and then we willposition it and resize it.This cause us trouble unknowingly.
When we manually position the table, the z-index of the 'style' attribut of 'table' tag is set to a greater value say 100.So the best thing is to write code for table and set the position at source side appropriately.Thats the reason for CalendarExtender not displaying some of the dates in calendar.You adjust the z-index of style attribute to 2 and check.
Hurray it works!
--- RaviShankar Kota
catch me at: http://visitravi.blogspot.com/