This is the 12673rd most frequent Polish word.
niedzielnych
"Of Sundays" or "Sunday-related."
Here, "niedzielnych" describes the afternoons (popołudni) related to Sunday.
Niedzielnych popołudni spędzamy w parku.
We spend Sunday afternoons in the park.
Here, "niedzielnych" is used figuratively to describe inexperienced drivers, often driving on Sundays for leisure.
Spotkaliśmy kilku niedzielnych kierowców na autostradzie.
We encountered some Sunday drivers on the highway.
Here, "niedzielnych" describes the religious services (nabożeństw) taking place on Sundays.