This is the 645th most frequent Croatian word.
petka
The Croatian word "petka" can mean "Friday" (colloquial) or "a grade of five" (highest grade in school).
Here, 'petka' is used to denote a set or pack of five items, common in colloquial usage.
Kupila sam petku olovaka.
I bought a pack of pencils.
In this sentence, 'petka' is used in the nominative case to mean 'Friday'.
U petak idem na koncert.
On Friday, I am going to a concert.
'Petka' here refers to the highest grade or mark (the number five) in the Croatian grading system.