Tridem

Tridem slides three pieces at once. A solid circle, a hollow circle and a diamond share the same board and the same move: an arrow, and all three go together in the same direction. Each piece has a destination square marked with dotted lines, and you must bring all three there. The game's clever idea lies in its calendar: the rule changes according to the day of the week, announced in a one-line notice above the board, for example 'only the diamond moves in a left-right mirror'. The banner shows the day's number, a rule name, a difficulty rating in stars, the move counter, the target par and your best score. The site states that each grid has a unique optimal solution, verified by its engine.

1. Read the rule line displayed above the board: it changes with the day of the week and determines how the three pieces respond to the arrows.
2. Move the three pieces in a single action using the keyboard arrows or WASD, or the four directional buttons shown under the board.
3. Bring each piece onto its dotted destination square, aiming for the par shown next to the move counter.

Moving three pieces in a single gesture is the kind of constraint that completely upends a sliding puzzle: you don't seek a single path, you seek a sequence that lines up three paths at once. The rule that changes with the day of the week adds a delightful cycle, the Monday puzzle doesn't play like the Tuesday one, and the par shown before the game gives a clear objective rather than a mere checkbox. One detail to know before starting: the published rules warn that the game's interface is in Japanese, even though the labels on the board are in English.

The board permanently displays three numbers: the number of moves played, the grid's par, and your best result, which is empty until you've solved it. No penalty is described for a bad move; four buttons handle the rest: Hint, Undo, Reset and Answer, which gives the solution away for those who give up. The day's ranking, shown under the board, is self-declared, the site itself says so.

Daily grid

One grid per day, numbered, with its rule dictated by the day of the week, its par and a difficulty rating in stars.

Archive

Date navigation above the board freely opens past days without an account. The site states there is a two-year stock of grids that do not repeat from one year to the next.

• The par is shown before you start: it's a design detail, not decoration. Finding a solution that uses many more moves than the par almost always means you missed the intended maneuver.
• Reread the rule line before touching the arrows. Because it depends on the day of the week, yesterday's reflexes are exactly what lose the first moves.
• Everything is on the keyboard: arrows or WASD to move, Z to undo, R to reset, H for a hint. Undo costs less than restarting.
• Past grids are open without an account via date navigation: a good training ground to discover the other rules of the week without waiting for the relevant day.