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The Rubik's Perpetual Calendar Cube was produced in the early 1980s, when Rubik-mania gripped the world.
The cube is physically identical to a standard Rubik's Cube, with the exception of the stickers. Instead of the familiar coloured squares, the faces are adorned with letters and numbers. By twisting and turning, the cube can be made to present a perpetual calendar on one face.
One sticker is marked "day" and is present for every daily solution. Other stickers are marked "Mon", "Tues", and so on, to make the days of the week. The month is spelled out in three large letters across the middle, and the bottom right two squares can be made to show the day of the month.
It's quite easy to solve the cube for any given day. Compared to the standard, coloured cube, the letters and numbers present the additional complication of requiring the correct orientation, but, crucially, only a single face needs to be solved.
The Rubik's Calendar is no longer available to buy new, but you can buy a set of stickers and convert an existing cube. I got my stickers (complete with layout instructions) from a company called Cubesmith. Applying them took some time (there are 54 of the little blighters), but was straightforward. The most fiddly part was removing the original coloured stickers; Cubesmith also sold me a useful scraping tool to help with this.
My Rubik's Calendar is one of my favourite possessions, and sits proudly on my desk, the envy of my co-workers. I love the technical elegance of the Rubik's Cube mechanism, and I love how it's an absurdly complicated, but perfectly functional, desktop calendar.