Pokemon Diamond and Pearl had less Pokemon appear during the anniversary of three well-known tragedies. Earlier this month, the code of Pokemon Diamond and Pearl leaked online, the result of an earlier Nintendo server hack. Fans combing through the source code have made several surprising discoveries, including that Pokemon are either more likely to appear or less likely to appear during certain real world dates. Notably, Pokemon are 10% less likely to appear on August 6th, August 9th, and September 11th, which are the anniversaries of the bombings of Hiroshima and Nagasaki during World War II and the anniversary on a terrorist attack in the United States that left nearly 3,000 dead. Pokemon are also less likely to appear during the Spring and Fall Equinox.
Other holidays, such as Culture Day, Christmas and New Year's Eve, also have decreased spawn rates, albeit at a smaller 5% rate. Although this feature seems to have been meant to possibly discourage players from playing during certain major holidays, these spawn rates also make an impact on speedruns for the game.
While Pokemon are less likely to appear during the anniversary of certain tragedies, dataminers also discovered that Pokemon are also more likely to appear during a number of different holidays, including St. Patrick's Day, the 4th of July, and Christmas Eve. Other countries' national holidays are also given boosted spawn rates, including Japan's National Foundation Day, Italy's Italian Liberation Day, and Germany's Volkstrauertag. A full list of the dates with boosted spawn rates can be found below.
While not a critical feature, this is definitely one of the stranger things we've seen in a Pokemon game. It seems that Game Freak definitely wanted to encourage players to play during certain National Holidays (which many players would have off) while also wanting to discourage players from playing on certain days.