Kôshin, or Kanoë-Saru, is one of the "Stem-Branch", the system of the Chinese calendar. There are 10 stems and 12 branches and each system makes its cycle, then the Stem-Branch System makes the 60 names' cycle.
10 Stems are the combination of Yin-Yang, or shade-light, and five elements; wood (ki), fire (hi), soil (tsuchi), metal (kane) and water (mizu). Each stem is called as follows:

	Name	Meaning		Name	Meaning
1	Kino-ë	Wood-Light	6	Tsuchino-to	Soil-Shade
2	Kino-to	Wood-Shade	7	Ka'no-ë	Metal-Light
3	Hino-ë	Fire-Light	8	Ka'no-to	Metal-Shade
4	Hino-to	Fire-Shade	9	Mizuno-ë	Water-Light
5	Tsuchino-ë	Soil-Light	10	Mizuno-to	Water-Shade

And 12 branches come from Chinese Zodiac:

	Name	Meaning		Name	Meaning
1	Ne(-zumi)	Rat	7	Uma	Horse
2	Ushi	Ox	8	Hitsuji	Sheep
3	Tora	Tiger	9	Saru	Monkey
4	U(-sagi)	Rabbit	10	(Niwa-)Tori	Chicken
5	Tatsu	Dragon	11	Inu	Dog
6	Mi	Snake	12	I(-noshishi)	Boar

Then, in Chinese calendar, each year is named with a specific stem-branch, so is each day. For example, the year 2000 is the year of Kanoë-Tatsu, and its 1st of January is the day of Tsuchinoë-Uma. The coming year of Kanoë-Saru will be the year 2040.

The belief of Kôshin originally came from a folklore based upon Chinese Daoism:
There is a insect called Sanshi in one's body, which looked at the bad deeds of its owner. On the night of Kanoë-Saru, when their owner is falling asleep, the Sanshis get out of their body and go to Ten-Tei, the Master of Heaven, to tell what they see. And Ten-Tei will punish the bad people by shortening their lives, sometimes even by killing. -- So those who believe this folklore stay awake all the night on Kanoë-Saru days to prevent Sanshi to get out of their body.
This belief was brought to Japan in Heian Era (794-1185) and became popular in aristocracy.

The custom of staying awake on Kanoë-Saru night was called "Kôshin-Machi", or Kôshin Waiting. In Heian Era, many Kôshin-Machi were held mainly in aristocracy, but they were nothing but overnight parties in fact.
In Yedo Era (1603-1867), Kôshin-Machi became very popular in folks and the form entirely changed. They also had Kôshin-Machi as overnight parties. In addition, they created two guardian deities of Kôshin.
One is the Shômen-Kongô. It is originally a demon which causes diseases. It is uncertain why Shômen-Kongô became the Kôshin deity, but the author guesses that folks expected that the demon to make sanshis ill and prevent them to go to Ten-Tei.
The other is three monkeys, each of which covers its eyes, mouth, and ears by its hands. They are called Mi-zaru (not see), Iwa-zaru (not say) and Kika-zaru (not hear), and the suffix "zaru" (not) is related to "saru" (monkey). It is also uncertain why these monkeys became the Kôshin deity, the author guesses that folks expected that the sanshis and the Ten-Tei not to see, say, hear the bad deeds of their owner.

Folks in Yedo Era made their symbol of Kôshin guardians. For example, they made a statue of Shômen-Kongô and put it in the temple, carved Shômen-Kongô, three monkeys, or letters "Kôshin" on stone blocks and put them around the area they lived.
And as a result, there are so many Kôshin-Tô (Kôshin Tower), Kôshin-Dzuka (Kôshin Mound), and Kôshin-Tô (Kôshin Temple), but the custom of Kôshin almost ceased to-day.
In addition, such custom caused a superstition that a person who was born on the day of Kanoë-Saru would be a thief, and you could prevent it by giving him/her the name related to gold, metal or money. For example, one of the most famous Japanese novelist Natsumé Sôseki was born on the day of Kanoë-Saru, so his real name was Kinnosuké ("Kin" means gold). To-day most of the people do not believe, do not even know, such a custom.