This is the 12831st most frequent Russian word.
купальской
"Of or related to bathing or the feast of Ivan Kupala."
Here, 'купальской' is used as an adjective to describe 'ночью', referring to 'Kupala's night', a traditional Slavic celebration.
В ночь на Купальскую ночь устраивают костры и прыгают через них.
On the night before Kupala's night, bonfires are lit and people jump over them.
The word 'купальской' again acts as an adjective modifying 'ночь', emphasizing the specific timing of the celebration.
Травы, собранные в Купальскую ночь, считаются особенно целебными.
Herbs collected during Kupala's night are considered particularly healing.
In this usage, 'купальские' relates to 'костюмы', indicating 'swimming costumes', showcasing a different nuance connected to water activities.