Where does “по́лдень” come from?

по́лдень (Russian) comes from Russian день, from Old East Slavic дьнь, from Proto-Slavic dь̑nь.

Ancestry of “по́лдень”, step by step

Step	Language	Word	Meaning
1	Russian	день	day, daytime; afternoon, early evening;...
2	Old East Slavic	дьнь	day
3	Proto-Slavic	dь̑nь	—