Historic is a synonym of Important. Important is related to Substantial. Substantial is a synonym of Wealthy. 
Gathering is a synonym of Assembly. Assembly is a synonym of Congress. Congress is related to Senate. Senate is related to Government. 
Historic <--Synonym--> Important <--RelatedTo--> Substantial <--Synonym--> Wealthy 
Gathering <--Synonym--> Assembly <--Synonym--> Congress <--RelatedTo--> Senate <--RelatedTo--> Government 