New Years Day. The first person to cross your threshold (walk through your door) had to have dark hair. So my father had black hair, and New Tears Eve after midnight he had to walk through twenty fucking apartments because not many Irish people had black hair. And it had to be a man not a woman.
That was a sign of good luck. Anybody else that came in, you weren’t gonna have a good year.
Since more English people have black hair, or the Irish people that do have black hair are the black Irish, I think that it might be a sign of welcoming guests/foreigners. Or perhaps a preventative magic because the English would have had black hair. Or, simply the fact that black hair is rare, and so it entails rare things (good things) for the rest of the year.