This is the 847th most frequent Croatian word.
prosinca
"Prosinca" means "December" in English, in the genitive case.
The word "prosinca" appears in the genitive case to specify 'of December' as part of a phrase describing a period.
Na kraju prosinca otputovat ćemo na odmor.
At the end of December, we will go on vacation.
Here, "prosinca" is used in the genitive case to denote 'of December,' specifying the date.
Božić se slavi 25. prosinca svake godine.
Christmas is celebrated on the 25th of December every year.
"Prosincu" in this sentence is in the locative case, indicating the time of occurrence.