#### Answer

Friday

#### Work Step by Step

Days on which first person lies = Friday, Saturday, Sunday
Days on which second person lies = Tuesday, Wednesday, Thursday
So, today is Friday, because if first person says that I lied yesterday which means Thursday, the day on which he doesn't lie, but its Friday so he can lie about yesterday which is correct.
Similarly, second person says truth on Friday, so if he says that I lied yesterday, means he is saying the truth today and lied yesterday which is Thursday, which is correct.
So, today is Friday.