Change from to-day to today

Solution 1:

Five minutes of research brings...

today
O.E. todæge, to dæge "on (the) day," from to "at, on" (see to) + dæge, dative of dæg "day" (see day). Generally written as two words until 16c., after which it usually was written to-day until early 20c.

Similar constructions exist in other Germanic 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."

The same applies to tomorrow and tonight, at least according to this dictionary.

Solution 2:

I grew up writing 'to-day' and 'week-end' (in 1950s' Britain). Common pairings of words seem, first, to be linked with a hyphen, then to become one word. I did not feel any differently about 'to-day' than I do these days about 'today'.