I'd personally like to know why name days are more important to some people than their birthdays.
Aren't you Irish, Teflcat? If so, just think St. Patrick's Day, it's like the whole country's nameday.
The namedays came from the idea of saint patrons. In older times people usually gave their children names of saints and the saint was supposed to protect and guide the baby through life, therefore the day associated with a saint was special for the person named after them.
There were lots of Catholic saints so practically every day of the year had a patron or two.
Also, a child often got the name which it "brought itself" coming to this world - meaning that a child born on St. John's day got the name John, and so on. This way, the birthday and the nameday were on the same day.
Another thing is that in older times the saints or church holidays were the usual way of telling time. Nobody knew when, let's say, 17th February was - in any case, it took some serious thinking and lots of calculating - but everybody knew St. Agnes' or St. Gregory's day.
The Protestant churches got rid of the saints, so just the birthdays remained. In Poland though, celebrating birthdays is a relatively new thing. Even my parents didn't do it when they were kids.
One more factor is that the official birth dates often did not correspond with the real ones. In my family, most of the aunts and uncles who were born shortly before the war, during the war or just after the war, have double birth dates - the real ones and the official ones, written in the documents. They were born at home and although there was an obligation to register a child within a given period of time, sometimes weeks passed before anybody had a chance to do it. And there were fines for not registering on time, so they didn't think twice about giving the registry office a wrong, later date. One of my aunts was born in October 1942 but the documents state January 1943.
Also, the namedays were convenient - you did not need to remember all the individual birth dates, but everybody in Poland knows when Barbaras or Stanisławs celebrate their namedays :)