truth on Saturdays, which means Kerry lies on Mondays, Tuesdays and
Wednesdays, and today is one of these days.
So when Kelly says I will lie tomorrow she is telling the truth as the
twins never lie on the same day.
So the day you seek is a day on which Kelly tells the truth (one of
Sunday, Monday, Tuesday, Wednesday), but will lie on the next day
(one of Thursday, Friday, Saturday).