We use any reason we can to justify a WDW trip, so birthdays are a great target for our trips. We celebrate. Anyway, we have had mixed results for these celebrations. For instance, my DGD turned 6 and we had a week long birthday party for her in WDW. That child wore her button everywhere! SOmetimes she was acknowledged with a birthday greeting, other times a cupcake. Sometimes nothing.
On my DH birthday, he had no idea that we were celebrating him that week, and was not at all comfortable being the center of attention, not even ours. The server heard us teasing him and the next thing you knwo the entire cast completed our meal by singing to him.
If you really want to celebrate, you should arrange what you want. If you want serendipity, see what happens. YOu just never knwo what will happen and if the staff is busy, odds are drastically reduced your fiancé will get even a cupcake. This is truely a pixie dust occurance, and not a promise.