Jón Ögmundsson (1052–1121), also known as John of Holar and Jon Helgi Ogmundarson, was an Icelandic bishop and local Icelandic saint. In 1106 he founded the episcopal see at Hólar in the north of Iceland and served as bishop there until his death.
A religious purist, Jón made it his mission to uproot all remnants of paganism. This included changing the names of the days of the week. Thus óðinsdagr, "day of Odin", became miðvikudagr, "mid-week day" and the days of Týr and Thor became the prosaic "third day" and "fifth day".
Jón's names for the days are still in use in Iceland today but despite the success of this cosmetic reform it appears that Jón did not manage to uproot the memory of the heathen gods. More than a century after his death the Prose Edda and Poetic Edda were written, preserving large amounts of pagan myth and poetry.
Jón was made a local saint by episcopal decree at the Althing in 1200.