How can I mark the days of the month on this D3JS heat map?

Question

I really like the "Months> days (horizontally)" graph on this page:

http://kamisama.imtqy.com/cal-heatmap/

I am absolutely new to D3JS, although I am wondering if it is possible to mark the day of the month (i.e. 1,2,3 ..., 31) directly in the calendar HTML (and not through the tooltip), I just want to create a calendar (with marked days) using D3JS.

How would I do that? Thanks!