When are you considered in delay? When are you liable for damages? (Articles 116-1170, Civil Code)

thank you for this, helps me alot in our subject OBLIGATION and CONTRACT

Kailangan po ba mag serve ng demand letter si creditor kay debtor kung nakasulat sa kontrata ang "the debtor should pay on _____ without demand". Yung Article 1169 sana po yung questiion ko.