It occurred to me about a year ago that the English-speaking world is counting birthdays wrongly. On the day that a person turns one year old, most of us are in the habit of saying that it’s their first birthday.
This is wrong.
Surely their first birthday was when they were born and were only a few seconds old. If that isn’t a birthday, I don’t know what is. My daughter is one year old tomorrow. I won’t be making a fuss if anyone calls it her first birthday, but I admit that when she was born I was pedantic enough to wish her happy birthday.
On the other hand, perhaps it’s not our counting that is wrong but the word birthday. The French use Anniversaire, which, like a wedding anniversary, doesn’t include the initial event.