There are certain days coded into the game's files that alter Pokemon spawn rates.

On August 6th, August 9th and September 11th, Pokemon are 10% less likely to spawn in the wild. These dates coincide with the atomic bombings on Japan, and the September 11th terrorist attacks on the United States.

On certain major holidays such as Christmas, New Years and Culture Day (November 3rd in Japan), Pokemon are 5% less likely to spawn in the wild possibly to discourage gameplay during holidays.

33 other holiday dates were found to have a 5% increase in wild Pokemon spawns, such as St. Patrick's Day (March 17th), Independence Day (July 4th in the United States), Bank Holiday (August 28th in the UK) and Christmas Eve (December 24th).