O.E. todæge, to dæge "on (the) day," from to "at, on" (see to) + dæge, dative of dæg "day." Generally written as two words until 16c., after which it usually was written to-day until early 20c. Similar constructions exist in other Gmc. languages (cf. Du.
van daag "from-day," Dan., Swed. i dag "in day"). Ger. heute is from O.H.G. hiutu, from P.Gmc. *hiu tagu "on (this) day," with first element from PIE pronomial stem *ki-, represented by L. cis "on this side."