Isn't it true that...should be the question to make it grammatically correct..

Isn't it true = Is it not true = Is it false/wrong - Answer should be 'No,' otherwise it technically provides a different meaning to 'Is it true,' as yes to that is in agreement that the day is Monday, where in reality 'Isn't it true' is agreeing that it is not true that the day is Monday; indeed meaning that it can only be one of 6 other days!

But thats the way the world is. Many things are incorrect with todays English.