Adding event to date on month calendar

I have the calendar set up and all is working great. Right now I just have a button on the Month calendar page that says add event. The form opens and you need to select the date. I would like to add a button to each date square in the month calendar that is labeled new. You would click on the button and a modal would pop up and you would be presented with the event form with that date already filled out. How would I go about doing that? I have the modal working with Bootstrap but I am just not sure how to fill in the date for the user. Any help would be much appreciated.