Where does “петък” come from?

петък (Bulgarian) comes from Old Church Slavonic пѧтъкъ, from Proto-Slavic pętъkъ, from Proto-Slavic pętъ, from Proto-Balto-Slavic penktas, from Proto-Hellenic penkʷtos, from Proto-Indo-European pénkʷe — five.

Ancestry of “петък”, step by step

StepLanguageWordMeaning
1Old Church SlavonicпѧтъкъFriday
2Proto-SlavicpętъkъFriday
3Proto-Slavicpętъfifth
4Proto-Balto-Slavicpenktasfifth
5Proto-Hellenicpenkʷtos—
6Proto-Indo-Europeanpénkʷefive
Every word from Proto-Indo-European pénkʷe →Every word from Proto-Hellenic penkʷtos →Every word from Proto-Balto-Slavic penktas →