Not a quirk, they are two different things. You can't RSVP on the gig calendar day because it's not an event. There could, in theory, be several events on May 3.
You can get to the event by going to the calendar day and then clicking the event title. This is where you RSVP.
eg.
May 3 calendar day link - http://www.mikafanclub.com/calendar/1-community-calendar/day-2015-05-3
Brooklyn event link - http://www.mikafanclub.com/calendar/event/12-mika-verboten-brooklyn-new-york-3-may-2015/