I think the formal nights are on the first at-sea day and the evening when you depart Cabo. That should be the Monday evening and the Friday evening.
Usually when the booking is made the agent will tell the cruise line about any special events such as birthdays, etc. I'd get with the maitre'd on the first day and let him know about the birthday. They will show up at the table with a little cake and a rousing version of "Happy Birthday" sung by the waiters. As most of the staff seems to be from eastern Europe these days it often sounds like "Eppy Birzdey Two Shoes".