THE DAY
September 9 is International Sudoku Day.
The date is a playful nod to the classic 9×9 Sudoku grid: the ninth day of the ninth month. The World Puzzle Federation established the observance in 2013.
THE NAME
“Sudoku” is a shortened Japanese name.
The puzzle was introduced in Japan in 1984 under the phrase Sūji wa dokushin ni kagiru, commonly translated as “the numbers must remain single” or “the numbers must occur only once.”
THE BIG NUMBER
There are 6,670,903,752,021,072,936,960 completed classic grids.
That is about 6.67 sextillion valid completed 9×9 Sudoku grids, counting grids that differ by number labels or allowed rearrangements as different grids.
THE MINIMUM
17 clues can be enough.
For a standard 9×9 Sudoku with a unique solution, 17 is the fewest number of given cells known to be possible — and exhaustive computation established that no 16-clue puzzle can have a unique solution.
THE CHAMPIONSHIPS
Sudoku became a competitive puzzle.
The first World Sudoku Championship was held in Lucca, Italy, in March 2006. Its competition included classic Sudoku as well as many variants.
THE LOOK
Symmetry became part of Sudoku's visual language.
Japanese publisher Nikoli helped establish the elegant, often symmetrical placement of the starting clues that became characteristic of many published Sudoku puzzles.
MORE TO COME...
Sources & further reading: World Sudoku Championship · University of Edinburgh — Sudoku modelling and solution · Nikoli — Sudoku