In the terms 'today, tomorrow, tonight' , what does 'to' mean?

It is a prefix that in some cases (like today) has survived from Middle English usage in words with reference to time meaning on (this day): Today: Old English todaege, to daege ''on (this) day,'' from to ''at, on'' (see to) + daege, dative of daeg ''day'' (see day). Meaning ''in modern times'' is from c. 1300.