This is the 12672nd most frequent Polish word.
niedzielnego
"Of Sunday" or "Sunday's."
Here, "niedzielnego" functions as an adjective in the genitive case, describing the afternoon as related to Sunday.
Wyszedłem na spacer w niedzielnego popołudnia.
I went for a walk on a Sunday afternoon.
In this context, "niedzielnego" serves as an adjective in the genitive case, attributing the tranquility to Sunday.
To była atmosfera niedzielnego spokoju.
It was the atmosphere of Sunday tranquility.
Here, "niedzielnego" acts as an adjective in the genitive case, modifying the performance to indicate it occurred on Sunday.