Lion said: Yesterday I was lying and two days after tomorrow I will be lying again.
Which day did he say that?

This conjunction is true only if both parts are true. The first part is true only on
Thursday, but the second part is a lie then (Sunday is not a lying day of the Lion). So
the whole statement is not true (at least one part is not true) and could be said only on a
lying day. Since the second part is a lie on any lying day, the Lion could have made the
statement on Monday, on Tuesday and even on Wednesday.