This Perpetual Calendar was made by the French company Sculptures-Jeux in 1991. It allows you to find out which day of the week an event happened, anywhere between 1600 and 2999. You just need to know the date.
There are 3 cylinders, which you rotate, in order to set the month, the year and the century. You then read off the day of the month on the left, and follow the line across the 3 cylinders, and it will lead you to one of the days of the week on the right.
So, if you want to know what day of the week it was on July 20th, 1969 (which was the day that Neil Armstrong walked on the moon) well, you set the dials, and discover... it was a Sunday. No batteries required, and it can even cope with Leap Years!