Omens

 Rabbeinu Yonah (Sefer HaYirah) And we should not assign any sign except what the Sages said, and if a person has a mitzvah to perform, such as reviewing  a gemora or starting  his son’s education, he should not say, "I will wait until Rosh Hashanah or until Pesach"... And those who are not concerned with omens will not be influenced by them, but those who are concerned, Satan stands at their right hand to oppose  them and strengthen  them in their error.