There is no such thing as a "good day" or a "bad day" for conducting a marriage. Those who believe that events unfold according to one's destiny cannot simultaneously believe that a specific day will bring good fortune while another brings bad luck.
It is not true that a marriage conducted during the waxing phase of the moon will be prosperous, or that one conducted during the waning phase will decline or fail. There is absolutely no basis for such beliefs.
We witness firsthand many marriages that end in divorce, despite having been scheduled on "auspicious" days during the waxing moon. This serves as further evidence that such beliefs are merely superstitions.
The Arabs living during the time of Prophet Muhammad (PBUH) believed that the month of Shawwal was inauspicious and would not hold significant events, such as weddings, during that month.
Regarding this, Aisha (RA) stated: "Prophet Muhammad (PBUH) married me during the month of Shawwal and consummated the marriage during that same month. Who among his wives was dearer to him than I?"
Source: Muslim 2551, Nasa'i 3184
Therefore, a marriage can be conducted in any month, on any day, and at any time. Labeling a specific day or time as "bad" and shunning it constitutes a grave offense.
Prophet Muhammad (PBUH) narrated that Allah says: "The son of Adam reviles Time, but the one who reviles Time reviles Me. I am Time; the authority lies in My hands. It is I who cause the alternation of night and day."
Narrator: Abu Hurairah (RA)
Source: Bukhari 4826, 6181, 7491
Thus, if one labels or believes a day to be "bad," one incurs the sin of attributing that negativity to Allah Himself.