Sanhedrin 65b: Rabbi Akiva says: The type of forbidden superstition called “m’onen” means the practice of saying that certain days are luckier than others to embark on a trip, or certain days are luckier to buy merchandise.
סנהדרין סה ע”ב תנו רבנן: מעונן… רבי עקיבא אומר: זה המחשב עתים ושעות, ואומר: היום יפה לצאת, למחר יפה ליקח.
יו”ד קע”ט ב’ נהגו שאין מתחילין בב’ ובד’ ואין נושאין נשים אלא במילוי הלבנה.
רמ”א אה”ע ס”ד ד’ ונהגו שלא לישא נשים אלא בתחילת החדש בעוד שהלבנה במלואה.
A chassidishe Yid asked Rabbi Yisroel Reisman: “Is it allowed to make a chasunah on the 28th day of Sivan? That’s the only day I could find a hall available. Usually we don’t make chasunos during the second half of the month, but perhaps Sivan is different.”
“I don’t know,” replied Rabbi Reisman, “but I’ll tell you a Hamakneh, written by the Baal Haflaah, Rabbi Pinchus Horowitz, who was chassidish. At the end of the sefer, in his comments on Even Hoezer 64, he asks why the Rema needs to say in Orach Chaim 551:2 that after the 17th of Tammuz we don’t make chasunos – isn’t that the second half of the month? He answers based on a teshuva from the Rema that the rule is that wherever he says נוהגין it means poskim taught that we should do so, but נהגו means that people just do it and we don’t stop them. Here, in the case of getting married in the second half of the month, the Rema (as well as the Mechaber in Yoreh Deah) says נהגו – we allow people to keep this custom. But the Rema never says that one is obligated to keep this custom.
A while later Rabbi Reisman met the man again and asked, “So when is the chasunah?” He replied, “Tu B’av.”
[It would seem that the Mechaber and Rema are telling us that it is allowed to keep this custom and there is no problem of לא לעוננו (superstition). The reason why it is not considered ניחוש and לא תעוננו is because we do it as a good sign, just as we anoint kings next to a spring (Beis Yosef, Yoreh Deah 179; Aryeh Devei Ilai Even Hoezer 18). A good sign is something that we aren’t particular about, but we just do to express our prayers that things will go well. A superstition, on the other hand, would mean refusing to get married in the second half of the month no matter what.] �