Sunday: the sun's day, and the sound of "son"

Sunday is the sun's day. Old English Sunnandæg translated the Latin Solis dies, "day of the sun", just as Mōnandæg translated the moon's day. German Sonntag and Dutch zondag match it. The Romance languages renamed it for the Christian "Lord's day", dies Dominica — French dimanche, Spanish domingo.

The vowel has moved. In Old English the U of sunne was a short "oo", like the vowel in put. In the 1600s, in southern England, that short vowel split in two: in some words it stayed (put, full, bull), and in most it opened to the "uh" of modern sun, cup and but. American English inherited the split.

The same change caught son, which scribes had long spelled with an o — medieval handwriting wrote u, n and m as rows of identical vertical strokes, and an o next to them was easier to read. So sun and son, Sunday and Monday, now share one vowel spelled two ways.