Who lied??

Lion lies on : MON TUE WED

RAT lies on : WED THURS SAT

If lion says : I didn’t lie yesterday

RAT says : I didn’t lie yesterday

so what day is today?

Answer:

Tuesday....

The statement "I didn't lie yesterday" indicates:

1. If X is lying today, X was lying yesterday

or

2. If X is not lying today, X didn't lie yesterday

Answer is Tuesday ( Lion is lying, Rat is speaking the truth).