party convention
Refers to a formal gathering of a political party, such as a national convention or congress.
党大会で新しい政策が発表された。
The new policy was announced at the party convention.
党大会 refers to a large, often annual party convention, while 党議 covers smaller internal party meetings or councils and their decisions.
extraordinary party convention
Compound of 党 (party) + 大会 (convention, mass meeting).