Where does “назавтра” come from?

назавтра (Russian) comes from Russian за́втра, from Old East Slavic завътра, from Proto-Slavic *za utra.

назавтра (Russian): the next day

Definitions

Step	Language	Word	Meaning
1	Russian	за́втра	—
2	Old East Slavic	завътра	—
3	Proto-Slavic	*za utra	—

назавтра — Etymology, Origin & Word History | Etymology Explorer