I read that po can take both the acc. and loc. 1) po (+acc.) = after; as far as, up to 2) po (+loc.) = after; along, over (the surface of). And both cover the sense of "after".
"piątek" (nom. masc. sg.). -> "piątek" (acc. masc. sg.) -> "piątku" (loc. masc. sg) (NB I couldn't find a declension of piątek anywhere, had to work this out from the rules)
Therefore the process of elimination indicates that the second meaning of "po" is in play here.
This makes semantic sense. The use of accusative with prepositions usually indicates motion, but since Friday and Saturday are "static", ie. neither is ever likely to move around to another position in the week., and Saturday is certainly not moving to a new position after Friday, then it makes sense that we are using "po"in the locative.
Usually after „po” you use locative. The two situations where you would use accusative is "slightly above" (example: „woda po pas”) or with objects you intend to take from somewhere („iść po klucz”).
I would suggest using the Polish Wiktionary. The English one is more error-prone it seems.
PS: Good just inflecting „piątek” correctly.
I think , when we are discussing time, because prepositions have so many meanings:
Za is after amount of time in the future. The best way to show this meaning is
9.50- ten minutes to ten - za dziesięć dziesiąta.
po is after something happened.
you are right za takes accusative here, while po locative.