In "Game of Thrones Season 1, Episode 4", Samwell Tarly explains to Jon Snow why he ended up taking the black (joining Nightwatch) as follows:
On the morning of my 18th Nameday, my father came to me. "You're almost a man now," he said. "But you're not worthy of my land and title. Tomorrow, you are going to take the black, forsake all claim to your inheritance and start North. If you do not," he said...
According to this Wikipedia article on 'nameday', it is
a tradition in some countries in Europe and Hispanic America that consists of celebrating a day of the year that is associated with one's given name. The celebration is similar to a birthday.
According to the same article, the list of countries where it is celebrated doesn't include any countries whose official language is English.
What is etymology of "nameday"? Is "nameday" synonymous with "birthday" in English? If not, what does it mean in contemporary English?