How to set a default value in a text box

I have a gridview that looks to a textbox for the parameter to pass to my stored procedure in SQL.  I would like to have the textbox default when the page loads to yesterdays date so the gridview will run with yesterdays date as the parameter, but I also need to be able to re-run the stored procedure when the user changes the date in the textbox.  I don't know how to set the textbox to default to yesterdays date except for using the pageload event which obviously means I can only use yesterdays date because whatever date is typed in is just reset when the page reloads.  Can someone please help me with this?  I know very little about asp.net and visual studios.