This category of puzzle, which I’m quite fond of, requires the player to complete a 9 x 9 grid of numbers in such a way that each digit appears only once in each column, row, and 3 x 3 subgrid. The number and positions of digits you’re given at the beginning have a lot to do with how difficult any given sudoku is to solve. (I go for the maximum level of difficulty, myself, because I enjoy the mental challenge.) So if you had to pick one day of the year to designate as International Sudoku Day, what date would spring instantly to mind? Well, the World Puzzle Federation, back in 2013, decided that the ninth day of September (9 x 9) was the only appropriate day, and conveniently, that works regardless of the order in which your country renders months and days.
Image credit: <a href="https://www.flickr.com/photos/johnjack/">Random McRandomhead</a> [<a href="https://creativecommons.org/licenses/by/2.0/">CC BY 2.0</a>], via <a href="https://www.flickr.com/photos/41541935@N00/4264107415">Flickr</a>