International Sudoku Day takes place, of course, on September 9 (9/9). Sudoku enthusiasts will no doubt notice the date’s symbolism. The goal of the Sudoku game is to fill a 9×9 grid so that each column, row, and 3×3 sub-grid contains all the digits from 1 to 9. So nine-nine is the natural choice for the day of celebration. It’s ...