How do I get info back from a pop up

Hi I have a calendar pop-up form, using system.web.ui.webcontrols.calendar. Nothing complicated. Its written in C#. I launch the pop up with javascript, But I can't figure out how to get the selected date back to the main web page. I have a text box on the main form just waiting to be populated with the selected date. I have no idea how to get it there. Thanks in advance. m