There have been several well-known people called John Day:

• John Day (fl. 1497), English merchant trading with Spain, who provided the most detailed account of John Cabot's voyage of 1497
• John Day (c. 1522-1584), Printer
• John Day (1574-c. 1640), Dramatist
• John Day (1738-1808), Fellow of the Royal Society

There is also a town in Oregon named John Day and two rivers in Oregon named the John Day River: One a tributary of the Columbia River in northeastern Oregon and one a smaller tributary of the Columbia River in northwestern Oregon.