David Always Tells The Truth On Mondays...

Another point of view is:

David will only claim to tell the truth tomorrow if he behaves the same today as he does tomorrow. The only two consecutive days on which David behaves the same are Tuesday and Wednesday. Therefore, it must be $\boxed{\text{Tuesday}}$ .

David could not have made the statement on Monday, Thursday and Saturday, the days he tells the truth because he lies the day after and his statement would contradict. He could not have made the statement one day before the days he tell the truth, or else he would have told the truth when he should lie. Therefore, he would not have made the statement on Sunday, Monday, Wednesday, Thursday, Friday and Saturday. What is left is $\boxed{\text{Tuesday}}$ and he lied.

By saying "I will tell the truth tomorrow". If he is telling the truth, then David must need two consecutive days where he will be honest ( today and tomorrow). But since there is no consecutive days where he will tell the truth (Tuesday, Thursday & Saturday). Then therefore he must be lying, and in order to make that statement, he must have said on a day where the next day he can still lie, and the only day possible is on a Tuesday. This is because Tuesday and Wednesdays are the only consecutive days he can lie.

Because he has decided to tell a lie theirfore he is telling alie and two consecuitive days when he can tells a lie are tuesday and wednesday.so it is TUESDAY

Since non of his truth telling days are consecutive, that mean this statement of his will be a lie .. meaning it'll be said on his lying days .. Only Tuesday and Wednesday are consecutive in his lying days so it must be "Tuesday" otherwise he'll be telling the truth in his lying days which will be a contradiction