The Korea Times had a piece a couple of days ago on Korea’s ‘five-day markets’ (오일장) which are markets (as you may have guessed) that are held every five days. It was interesting to hear that these still exist in modern Korea so long after the society which created them and needed them met with its demise.
As the article points out, these markets first arose after the Japanese invasions of the peninsula in the late 16th century, usually regarded by historians as something of a watershed in the history of the Chosôn dynasty. The point of this system, as far as I remember, was that the markets were always within a days' walk of each other and farmers or traders (pobusang perhaps) could circulate around them, tramping from one to the next, presumably covering five different markets every five days.
This reminds me of the (very) famous Korean short story 'When the Buckwheat Blooms' (메밀꽃필무렵) by Kasan Yi Hyosôk, in which a band of peddlers make an overnight journey from one village market to the next. There was an English translation in the Autumn 1999 issue of Koreana (not online yet unfortunately) and it shouldn't be too hard to find in Korean, I would guess that it's probably in a lot of high school Korean textbooks. Here's an extract from the beginning (translated by Bruce and Ju Chan Fulton):
Every peddler who made the rounds of the countryside markets knew that business was never any good in the summer. And on this particular day, the marketplace in Pongp'yông was already deserted, though the sun was still high in the sky; its heat, seeping under the awnings of the peddlers' stalls, was enough to sear your spine. Most of the villagers had gone home, and you couldn't stay open forever just to do business with the farmhands who would have been happy to swap a bundle of firewood for a bottle of kerosene or some fish. The swarms of flies had become a nuisance, and the local boys were as pesky as gnats.
"Shall we call it a day?" ventured Hô Saengwon, a left-handed man with a pockmarked face, to his fellow dry-goods peddler Cho Sôndal.
"Sounds good to me. We've never done well here in Pongp'yông. We'll have to make a bundle tomorrow in Taehwa."
"And we'll have to walk all night to get there," said Hô.
"I don't mind - we'll have the moon to light the way."