The Rama (Rabbi Moshe Isserles of Cracow, Poland, 1525-1572), in a famous comment (Eben Ha’ezer 64:3), records a custom to make weddings only during the period of the month when the moon is increasing in size – meaning, the first half of the month. As the moon grows during this period, it is considered an auspicious time that bodes well for the marriage. Maran records this custom, as well, in Yore De’a (179:2), and many communities indeed follow the practice of ensuring to schedule weddings specifically during the first part of the month.
Different opinions exist as to when precisely during the month weddings should be held according to this custom. Some say weddings should be conducted through the 15th of the month, whereas others maintain that since 18 is an auspicious number (as it is the numerical value of "Hai"), weddings may be made through the 18th of the month. Yet another view allows weddings until the 22nd of the month.
Regardless, many authorities observe the widespread practice to allow weddings anytime during the month, and not to make a point of scheduling weddings during the first half of the month. And it seems clear that the accepted custom among Sepharadim is to be lenient in this regard and to allow weddings at any time during the month. This appears in the work Yehuda Ya’ale (Rabbi Yehuda Asad, Hungary, 1794-1866) and in the Aruch Ha’shulhan (Rav Yechiel Michel Epstein of Nevarduk, 1829-1908). It is also recorded in the book Pe’er Ha’dor (vol. 4, p. 200) that the Hazon Ish (Rav Avraham Yeshaya Karelitz, 1879-1954) was asked about scheduling weddings, and he answered explicitly that the custom is not to make a point of scheduling weddings at any particular time of month.
Of course, if the wedding can be easily scheduled for the first half of the month, then this is preferable. But couples should not avoid making their weddings in the second half of the month if this would cause any sort of complication, such as if the bride would be a Nidda at the wedding, or if this would cause some other practical inconvenience or difficulty.
Even among those who observe this custom, they do so only for first marriages. When it comes to second marriages, they allow weddings during the second half of the month, just as several other customs are not observed at a second wedding (such as wearing a long white gown). Furthermore, this custom is not observed during the month of Elul. Every day throughout the month of Elul is considered special and an auspicious time for a wedding, and therefore even those who normally insist on scheduling weddings specifically during the first half of the month would allow weddings during the latter part of Elul. This applies to the month of Adar, as well, according to many opinions.
Halachically speaking, it is permissible to make a wedding during the Aseret Yemeh Teshuba (the period between Rosh Hashanah and Yom Kippur). As a practical matter, this might not be an especially convenient time for a wedding, as it is a very busy time of year, but from a Halachic standpoint, weddings may be held during the period between Rosh Hashanah and Yom Kippur.
Summary: Some communities have the custom to schedule weddings specifically during the first half of the Hebrew month, but the more common practice is to make weddings whenever it is convenient for the people involved. A couple should certainly not insist on getting married during the first half of the month if this poses any sort of complications or problems.
