Both happy and sad.
Honestly, we’ve seen our fair share of leaks in regards to Pokémon. But recently, the source for Pokémon Diamond & Pearl got leaked. And it’s been a hacker’s wonderland.
But notably, they’ve found that certain days affect spawn rates and other little things. For example, one date that Junichi Masuda himself stated has something attached is his birthday, January 12th. Mr.Cheeze, well known for getting into the nuts and bolts of games, found that on that day in game, eggs the player is trying to hatch grow 10% faster.
But that’s far from the only day to feature some rate changes (obviously). Most well known date-specific holidays have some sort of increase or decrease to spawn rates.
But they also altered the encounter rates down by 5% for a few holidays; specifically, Christmas and New Year’s Day. They’re also decreased by 10% on the anniversary of certain terrible events, like the bombing of Hiroshima and 9/11. They’re also decreased by that amount on the spring and autumn solstices, probably as a form of balance to the summer and winter solstices.
As to why they’d do this, though? Well, Orcastraw, a speedrunner, data-miner, and hacker, hypothesizes to Polygon that they did it due to the fact that these were the games to first include a true-to-time watch and calendar in the form of the Pokétch. The games used that feature to decide when it should snow, and other weather phenomena.
While the Pokémon games have continued to do things with day/night cycles (and had been since Gold & Silver), they haven’t seemingly done anything with dates since. All of these spawn changes are unique to the Gen 4 games.
Source: Polygon