Entering a date by clicking with a mouse on a pop-up calendar is a metaphor familiar to users form many pieces of software. This is a cross-browser version of that interface component. It can be triggered by clicking on the text field labelled "Day" (assuming a javascript capable/enabled browser with support for the features that the script employs, as with all browser scripts).
Entering a date into a form using a pop-up calendar strikes me as a very mouse orientated activity so I decided not to have the pop-up activated when the user tabs to the "Day" field. If they use the keyboard to navigate to the field in the first place they will probably prefer to type the date in rather than having to switch to the mouse (or tab heavily within the calendar to get to the button they want to activate.