Katniss and Peeta are liars. They both lie on some specific days.
Katniss lies on Fridays, Saturdays and Sundays. She speaks the truth on all other days.
Peeta lies on Tuesdays, Wednesdays and Thursdays. He speaks the truth on all other days.
Can you tell that one day of the week when both of them will say "Tomorrow, I will lie."?