This is the 16495th most frequent Polish word.
wielkanocnym
"Relating to Easter."
The word "wielkanocnym" here is an adjective derived from "Wielkanoc", which means "Easter". It is in instrumental case, modifying "stole" to describe the context of the activity associated with Easter.
Tradycyjnie zasiadamy przy wielkanocnym stole.
Traditionally, we sit by the Easter table.
"Wielkanocnym" is an adjective here, describing the basket ("koszyku") that is associated with Easter celebrations, and it is used in locative case to indicate where the action is occurring.
Dzieci szukały wielkanocnym koszyku czekoladowych jajek.
The children looked in the Easter basket for chocolate eggs.
In this sentence, "wielkanocnym" is an adjective referring to "doświadczeniu" ("experience") and is in instrumental case, linking the experience to the context of Easter.