how to disable previous date in the asp.net calender?

Hallo everyone..
can anyone assist me on how to disable previous date on the asp.net calender.

i create this application using microsoft visual web developer 2005 express edition..
the calender i expected is will automatically disabled the previous date.

eg. today is 23/3/2007. the calender will disable the all the date before 23/3/2007.

i am developing a ticket reservation system for my education project.
hope you all might assist me..

i appreciate a alot for your kindness..

THANKS:)
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