The earlier submission appears to be from Christmas Eve, when the entire world was on a weekend and most English-speaking countries were either on a major holid

The earlier submission appears to be from Christmas Eve, when the entire world was on a weekend and most English-speaking countries were either on a major holiday or preparing for its imminent arrival. This submission was late on New Years Day in the USA, at a time when much of the world was already halfway through Monday, January 2. (I believe this was submitted at noon on Monday Sydney time, for example.) That's just one of many alternative hypotheses to explain the difference in traffic. Obviously, there's also a raft of "competitive" hypotheses: Articles on HN don't sink or swim in a vacuum, they have to compete with the other articles on the site at any given moment.