The other instance was to Aqua, though you wondered if she remembered, or if Equestrian culture had any event based around a birthday. You hadn't seen any pony celebrate their birthday once since you arrived, with the exception of the Royal Sisters, although that probably had more to do with the fact that, like the Royal Guard, you were kept within the confines of the castle. Maybe the Royal Sisters' importance meant they were the only ones who celebrated birthdays?