This is the 16213th most frequent Polish word.
upalnego
"Upalnego" means "of a sweltering/heatwave day."
The word 'upalnego' is an adjective in the genitive singular form, describing the noun 'dnia' (day), indicating that it is 'hot'.
W upalnego dnia warto schować się w cieniu.
On a hot day, it is worth hiding in the shade.
Here, 'upalne' is an adjective for 'noce' (nights) in the accusative plural form, describing its characteristic as 'hot'.
Nie mogłem zasnąć w te upalne noce.
I could not sleep during those hot nights.
The word 'upalnego' functions as an adjective in the genitive singular form, modifying 'popołudnia' (afternoon), giving it the trait 'hot'.