Passing a string to a modal window

I am using a calendar control. Each cell in the calendar contains the 'title' of an event. When I select the daydate I can, using the event title, retrieve the rest of the event data from the database. I can successfully format that data into a string. I can successfully open a child modal window. Now I want to pass the formatted string to the modal window. I don't need to post any data back to the parent window - I just want to display the formatted string. It would be great to display it in a label on the child window but I don't know how to pass the string to a label in the child.

Any help would be appreciated.