The rule is simple, but all the work that led up to the formulation of it wasn’t :)

Why Thursday?

Presumably because then you know that the majority of week 1 is in the new year.

Yes exactly. Week 1 is the first week where the majority of the days in the week are in the new year. If Thursday is in the new year, so is also Friday, Saturday and Sunday and with those four days in the new year, it is always the majority of the week.

> If Thursday is in the new year, so is also Friday, Saturday and Sunday and with those four days in the new year, it is always the majority of the week.

That's only if you count Sunday as the last day of a week, instead of it being the first day of the following week.

ISO counts Monday as the first day of the week.