"Yesterday Was One Of My Lying Days,".
Riddle: Betty and Becky are twins, who love to lie on certain days. Betty lies on Mondays, Tuesdays, and Wednesdays, while Becky lies on Thursdays, Fridays, and Saturdays. One day, you come up to the twins and ask them what day it is. They say, "Yesterday was one of my lying days," in unison. What day is it?
Answer: It can't be Sunday because both girls would be telling the truth…and someone is obviously lying now. It can either be Monday or Thursday because on those days, either Betty or Becky lies. But if it's Monday, both girls would lie saying yesterday was a lying day. Therefore, it's Thursday; Betty is telling the truth, and Becky is lying.
Source: https://www.riddles.com/11441
