onclick

I have a problem with the drop-down box, I have a form that give ppl to input the birthday, in contain 2 drop-down box.

I want to make this page act like this:
if the ppl select 02 in the month drop-down box, the day drop-down box will immediately change the day drop-down box's choose, make it just have 1-29, and also, if ppl choose 01 in the month, it will have 31 in the day drop-down box!

I have make a function that do it, but I don't know how to activate this function to make this page act like that!

can anyone help! thx very much!