Modal popup update

I have a label at the top of a form that shows a month / year combination.

There is an image button that opens a modal popup window that you can select a different month/year in. Pressing OK closes the window. But in the

button click event I attempt to reset the label values to the new selection. 

first set a variable to month name : strmonth = ddlmonth.selectedvalue;

then the label : lblmonth.text = strmonth;

but the code in the OK clicked event never fires.