Gemara

(R. Ami): To know whether one will survive the coming year, he should hang a lamp during the 10 days of Teshuvah in a house without wind. If all the oil burns, this shows that he will live. If one wants to go on a trip and wants to know if he will return, he should stand in a dark house. If he sees a Bevo'ah d'Bevo'ah (a shadow of his shadow), he will return.

Rejection: He should not do this. Perhaps he will not see it, and he will worry, and this will spoil his Mazal (and cause that he not return).

Mo'ed Katan 28a (Rava): (Length of) life, children and wealth do not depend on merit, rather, on Mazal;

Rabah and Rav Chisda were both utterly righteous Chachamim. When either prayed for rain, it would come. Rav Chisda lived 92 years, and Rabah only 40. There were 60 marriages in Rav Chisda's house (of his children), and 60 miscarriages in Rabah's house. In Rav Chisda's house there was abundant fine flour even for dogs; in Rabah's house even barley bread could not be found for people.

Shabbos 156a (Rav): We learn from here that Mazal does not apply to Yisrael. "Va'Yotzei Oso ha'Chutzah" - Hash-m told Avraham 'you are concerned because Tzedek (your Mazal) is in the west (which is too cold to beget)' I will put it in the east!

On R. Yehoshua ben Levi's ledger it was written 'If one was born on Sunday, he will be complete in one (Midah), and he will not have another (trait)'.

This means that he will be completely good or totally bad. Rav Ashi and the head of the thieves were both born on Sunday.

If one was born on Monday, he will be irritable...

(R. Chanina): One's Mazal (dispositions) do not depend on which day he was born, rather, the hour he was born (the five closest planets, sun and moon alternate 'dominating' for one hour, according to a fixed rotation). If one was born when the sun is dominant, he will illuminate like the sun. He will eat and drink his own food and drink. His affairs will be exposed... If one was born under Mazal Venus, he will be rich and adulterous ...

Mazal applies to Yisrael. It causes people to be Chacham or wealthy.

(R. Yochanan): We learn that Mazal does not apply to Yisrael from "...El Derech ha'Goyim Al Tilmadu ume'Osos ha'Shomayim Al Tachotu Ki Yechatu ha'Goyim me'Hemah";

Astrologers told R. Akiva that the day his daughter gets married, she will be killed by a snake. He was very worried. The day of her Chupah she did a great Mitzvah. Later, she stuck a pin in the wall. It struck a snake and killed it.

Astrologers had told the mother of Rav Nachman bar Yitzchak that he will be a thief. She told him to always (properly) cover his head, so that fear of Shomayim will be upon him, and to request mercy (to be saved from the Yetzer ha'Ra). He did not understand why. One day he was learning under a date tree; his garment fell from his head. He looked up and saw the date tree (it was not his). His Yetzer ha'Ra overpowered him, he cut a cluster of dates with his teeth.

Yevamos 64b (R. Mordecai): One may not marry a Katlanis (a woman widowed twice), for Bi'ah with her causes her husband to die.

(Rav Ashi): Her Mazal causes him to die.

Ta'anis 25a: R. Elazar was exceedingly poor. He let blood and did not have what to eat afterwards. He fell asleep, and later told what happened;

R. Elazar: How long will I be pained in this world?!

Hash-m: Do you want Me to recreate the world from the beginning? Perhaps you will be born at a time (with good Mazal) for income.

29b (Rav Papa): If a Yisrael has a case against a Nochri, he should avoid going to Din with him in the month of Av, when the Yisrael has bad Mazal. He should strive to go to Din in Adar, when the Yisrael has good Mazal.

Rishonim

Rosh (Teshuvah 53:8): Husbands of a Katlanis die, for it is her Mazal to struggle financially. (Women do not go outside; a widow has no one to support her.) We do not find that there is a Mazal for a woman's husband to die.

Avnei Chefetz (62, cited in Otzar ha'Poskim EH 9:5:4): If a Nochris Katlanis converted, if it depends on Bi'ah, one may not marry her. If it depends on Mazal, conversion changed her Mazal for the better.

Piskei Din Yerushalayim (Dinei Mamonos 6 p. 272): If it depends on Mazal, it depends on the law of an ox Mu'ad (established) to gore Nochrim. Rashi holds that it is not Mu'ad for Yisrael, who have better Mazal than Nochrim. We permit the widow for the same reason. The Rambam holds that the ox is Mu'ad for Yisrael. We do not say that Yisrael have better Mazal than Nochrim. Likewise, we forbid the widow.

Nimukei Yosef (Sanhedrin 16b DH Tanu): Hash-m decreed to run His world through the constellations. A Chasid should pray to annul the decree of Mazal. Everything is from Hash-m, and He can change it like He desires.

(Tosfos Mo'ed Katan 28a DH Ela): There is Mazal, but it can change through great merits. However, sometimes it does not change, like we find regarding R. Elazar.

Question: If the opinion that Ein Mazal l'Yisrael says that it can change through great merits, R. Chanina must say that even merits cannot change it. The Torah promises rewards in this world for Mitzvos!

Answer (Maharsha 156a DH Ein): They argue only about individuals. The rewards are for Kelal Yisrael, all agree that Klal Yisrael is not governed by Mazal.

Ritva (Ta'anis 29b DH Gemara): Ein Mazal l'Yisrael in other months, but Hash-m decreed that Yisrael have bad Mazal in Av and good Mazal in Adar. Alternatively, Rav Papa does not truly refer to Mazal. People call decrees 'Mazal.'

Maharsha (29b DH b'Av): Ein Mazal l'Yisrael, but good things come on good days, and bad things come on bad days.

Poskim

Rema (YD 179:2): If one knows that something opposes Mazal, he should not do it and rely on a miracle. However, one should not investigate this due to "Tamim Tihyeh..."

Rashba (Teshuvah 283 attributed to the Ramban, in Beis Yosef DH Kosav ha'Rambam): One should not act against his Mazal and rely on a miracle. The Rambam says that all this is folly. The Sugya shows otherwise. These matters require investigation. One should not be too exacting with people about them.

Yam Shel Shlomo (Bava Kama 5:23): Three things depend on Mazal, but surely Hash-m causes one to have a good or bad Mazal based on his merits (that he will have, or merits of his ancestors) or transgressions!

Gra (7): We find that R. Akiva and Rav Nachman bar Yitzchak were afraid due to the astrologers' words.

Rema (OC 664:1): The Rishonim say that on the night of Hoshanah Rabah there is a Siman in the shade of the moon foretelling what will happen to one or his relatives during the year. Some say that one should not be particular about this, lest it ruin his Mazal, for many do not understand this properly. It is better to be Tamim than to delve into future events.

Source (Darchei Moshe 2 citing the Avudaraham): Horayos 12a says that one can test his Mazal similarly before embarking, but it is better not to.

Kaf ha'Chayim (18): A case occurred in which one who did not see the shadow did Teshuvah, and his Din was torn. This is possible because the Din is not executed until Shemini Atzeres.

Kaf ha'Chayim (19): One wraps himself only in a sheet and goes to where the moon shines. He takes off the sheet and spreads his fingers. If his head has no Bevo'ah d'Bevo'ah, this shows that he will die. If a finger of his right (or left) hand has no such shadow, this shows that one of his sons (or daughters) will die. One must do so at a time when the moon dominates.

Mishneh Halachos (5:212): Derech Pikudecha says that if based on nature, a woman cannot conceive, even if she bore children, her husband did not fulfill Peru u'Rvu, for he did not engage in proper Hishtadlus (efforts) to fulfill the Mitzvah. I say that he fulfilled the Mitzvah. Ein Mazal l'Yisrael. Everything about Yisrael is a miracle. Nature does not apply!