Why was "tantrum" trending last week?
Old English Sunnandæg, literally "day of the sun," from sunnan, oblique case of sunne "sun" (see sun (n.)) + dæg "day" (see day).
A West Germanic loan-translation of Latin dies solis "day of the sun," which is itself a loan-translation of Greek hemera heliou. Cf. Old Norse sunnundagr, German Sonntag "Sunday." Like other weekday names, not regularly capitalized until 17c. Sunday school dates from 1783 (originally for secular instruction); Sunday clothes is from 1640s. Sunday driver is from 1925.