Birthday is the opposite of Happy. Happy is related to Cheer. Cheer is a synonym of Assure. Assure is a synonym of Convince. 
Landmark is a synonym of Historic. Historic is related to Constitution. Constitution is a synonym of Organization. 
Birthday <--Antonym--> Happy <--RelatedTo--> Cheer <--Synonym--> Assure <--Synonym--> Convince 
Landmark <--Synonym--> Historic <--RelatedTo--> Constitution <--Synonym--> Organization 