accessing asp.net controls of a page from another pae.

I have a page with a button that when the button is clicked, a new window containing a calendar opens. I would like to set the value of a textbox in the main page to the selected date of the calendar page. How do I do that?
 on the codebehind file for the calendar i have
protected void Change_Date(System.Object sender, System.EventArgs e)
        {
            // I would like to set the text of dateTextBox from another page to the SelectedDate of this calender.
        }
 
Thanks.

...