This is the 326th most frequent Polish word.
partii
"Partii" is the genitive or locative singular form of "partia," meaning "party" (as in a political party) in Polish.
Here, 'partii' is the genitive singular form of 'partia', meaning 'party', referring specifically to a political organization.
Zapisałem się do partii politycznej.
I joined a political party.
Here, 'partii' is the genitive singular form and is used to describe the upcoming congress as belonging to or associated with the party.
W niedzielę odbędzie się zjazd partii.
On Sunday, there will be a party congress.
In this instance, 'partii' is used in the genitive singular to denote possession or association, specifically to this party's actions.