The day of the week in which a market is held in some particular location(noun)
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Use "market day" in a sentence
"It was Satturday wch is their market day and there was a great Market Like some little ffaire, a great deale of Corne and all sorts of ware and ffowles there."
"I was there on Satturday wch is their market day and saw they sold their Butter by ye pinte 20 ounces for 6 pence and often for 5d or 4d; they make it up in a Mold just in the shape of a pinte pot and so sell it."
"A mile or two beyond this Ullswater, – Tuesday is the market day wch was the Day I came thither."