OnlineSudoku.io Daily Sudoku

OnlineSudoku.io is a free browser puzzle site built around sudoku, with killer, jigsaw, samurai and mini variants and a set of other logic games beside them. Its Daily Sudoku Challenge is one page with one classic 9x9 board for the day, titled with the date, so everyone who opens it that day solves the same grid.

The board sits in the middle of the page with a number pad, an undo button and a print button. Rows are lettered A to I and columns numbered 1 to 9. A timer runs while you solve, the score goes up by 30 for each correct digit, and a wrong digit shows in red and counts against a limit of three mistakes. Above the board a countdown shows how long until the next puzzle, which arrives at midnight UTC.

The level is set by the day of the week. Monday's board is easy, Tuesday and Thursday bring a medium grid, Wednesday and Friday a hard one, and the weekend boards are expert. The page's code builds each board from the date, so a reload brings back the same puzzle.

A date picker and a Previous day button load the board of any earlier day, and View Archive lists past puzzles. The page keeps your current streak and best time without an account, and a language menu switches the site into more than a dozen languages. The site is free and carries Google ads, with a slot beside the board and automatic ad units elsewhere on the page.

It suits players who want one plain classic sudoku a day that gets harder toward the weekend, and anyone who likes to catch up on days they missed.