Astrology

 Shulchan Aruch (Y.D. 179:1–2): 1) It is prohibited to ask about the future from an astrologer or by using a divination technique. Rema: Because of the prohibition of be tamim with your G-d  (Beis Yosef citing Tosfos based on the Sifri) and it is surely prohibited consult with magicians and diviners of all types. 2) The accepted practice is not to start something new on Monday or Wednesday and not to get married except while the moon is increasing [i.e., first half of the month]. Rema: This is why it is customary to start the new period of Torah learning on Rosh Chodesh because even though we don’t consult divination we do utilize propitious signs. If a person knows that some activity is against mazel he should not do it and have to rely on a miracle. That is because the prohibition is only to seek out this information because of be tamim with your G-d  [but one should utilize the information if he has acquired it].