add a class to all day-divs when loading calendar

add a class to all day-divs when loading calendar

i need to add the class 'modalbox' to all div's representing a day. With
that class I want to trigger a pop-up form.
Where or how do $( ".fc-day").addClass("modalbox");
PS I am just a snippet guy trying and putting all kind of things together...
Thx in advance