Direct borrowing from Dutch 'zondag'. Used in historical or regional contexts, especially in Nagasaki where Dutch influence was strong. Not standard modern Japanese for Sunday.
from Dutch zondag
Extended meaning from Sunday to any holiday or day off. Mainly survives in the name of the Hakata Dontaku festival (博多どんたく). Not used in everyday conversation.
In Nagasaki, they used to call Sunday 'dontaku'.
Hakata Dontaku is a big festival held every May.