The Rama (Rabbi Moshe Isserles of Cracow, 1525-1572) mentions a practice observed by some Ashkenazim to try to schedule weddings specifically during the first two weeks of the month, rather than the last two weeks. During the first half of the month, the moon is growing, and it is believed that this is therefore an auspicious time for a couple to get married, as it is a time of Mazal (good fortune). During the second half of the month, the moon’s size steadily diminishes, and this is thus an inauspicious time for a wedding. For this reason, some Ashkenazim endeavor to schedule their weddings specifically during the first two weeks of the month.
Sepharadim, however, do not follow this practice. For Sepharadim, all days of the month are equally suitable for a wedding, and there is no preference at all given to the first half of the month over the second half.
It should be noted that even among Ashkenazim, the preference to schedule a wedding during the first half of the month does not override other important considerations. Thus, for example, Rav Yosef Shalom Elyashiv (contemporary) ruled that if a woman’s cycle is such that she would be a Nidda during the first half of the month, then the wedding should certainly be scheduled for the second half. The interest in avoiding a wedding in a state of Nidda overrides the practice of getting married in the first half of the month. This is especially so in light of the ruling of the Rambam (Rabbi Moshe Maimonides, Spain-Egypt, 1135-1204) disqualifying a wedding held while the bride is a Nidda. Although we do not follow the Rambam’s ruling, it is preferable to avoid this situation, and this preference overrides the preference in Ashkenazic communities to get married specifically in the first half of the month.
Regardless, as mentioned, for Sepharadim this is not an issue at all, and all days of the month are considered equally suitable and auspicious for weddings.
Summary: There is a custom among Ashkenazim to schedule weddings specifically during the first half of the month when circumstances allow, but Sepharadim do not follow this custom, and all days of the month are equally suitable for weddings.