Thread:Vimitsu/@comment-27847604-20160317074144/@comment-24162773-20160320010826

The anime states that Rikka's birthday is June 12, as we see in the first 30 seconds or so of Season 2, Episode 6, where it is marked on the calendar, which is turned to the month of June (denoted by the 6 at the top). Link: here.