This is the 257th most frequent Polish word.
grudnia
"Grudnia" means "December" in English (genitive case in Polish).
'grudnia' is the genitive form of 'grudzień,' used here to denote a specific day within the month.
Pierwszego grudnia padał śnieg.
On the first of December, it was snowing.
Here, 'grudnia' is used to indicate a deadline by the end of the month.
Księgowa zakończyła raport do ostatniego grudnia.
The accountant completed the report by the end of December.
In this example, 'grudnia' indirectly refers to the time period related to this month, though the usage is informal and less standard.