Data miners have discovered spawn rates in Pokémon Diamond and Pearl appear to change on significant dates.
Pokémon GO is a unique title for a number of reasons. Not only can its Pokédex be tweaked and added to whenever Niantic chooses to do so, but things like spawn rates can be adapted to respond to the weather, the time of day, and what the date is. However, it would appear that all of these "new" features aren't as unique as players first believed.
According to various data miners, but still not confirmed by Nintendo or The Pokémon Company, spawn rates in Pokémon Diamond and Pearl change depending on the date. Pokémon speedrunner and shiny hunter @shinyhunter_map appears to have been the first player to have made the discovery. Their first discovery was that Pokémon spawn rates are increased by 10% on game developer Junichi Masuda's birthday.
Masuda would always encourage players to fire up Diamond and Pearl on his birthday, promising that something would be different. However, it wasn't until now that players may have figured out what that something was. A little more digging and it seems the spawn rate changes run far deeper. On days like St. Patrick's Day and Independence Day, the spawn rate jumps up by 5%.
However, there are a handful of days where the spawn rate goes down. September 11 and the anniversaries of the bombings of Hiroshima and Nagasaki all make that list. According to the source code discovered, spawn rates drop by 10% on these days. Apparently there is also a 5% decrease on Christmas Day and New Year's Day, presumably because the developers assumed people would be busy doing other things on those holidays.
Had players not delved into Diamond and Pearl's code, this might have been a detail that was never picked up on. It certainly seems to be something that has gone unnoticed until now. Players who felt as if they were stumbling upon more or less Pokémon will have chalked it up to having a good day, or a bad one. Again, no one officially involved in the production of the games has confirmed any of this to be true. It also doesn't seem to be a feature that has hung around for more recent Pokémon titles.
NetEase called it a bug, but others called it a feature.