Getting married on Ḥol Ha-mo’ed is forbidden, as we do not mix two joyful occasions together. There is already a commandment to rejoice on the festival, as we read: “You shall rejoice in your festival” (Devarim 16:14). It would dilute the joy of the festival if we combined it with the joy of another celebration. Newlyweds are so focused on enjoying each other’s company that they would likely neglect the festival. Additionally, the tremendous amount of work involved in organizing a wedding and setting up a household would likely detract from the joy of the festival. Furthermore, the Sages were concerned that if marrying on the festival were permitted, couples would push off getting married until the festival and thus delay fulfilling the mitzva to be fruitful and multiply. A festival wedding would be doubly appealing: more people would likely be able to participate in the celebration, and the couple could save money by combining the festival meal and the wedding meal (MK 8b).
Not only are first marriages prohibited on Ḥol Ha-mo’ed, but so are second marriages, as they too involve great joy. However, a divorced couple who decide to remarry each other may do so on Ḥol Ha-mo’ed, as their marriage does not involve such great joy (SA 546:1-2).
One may get married the day before a festival and recite Sheva Berakhot at the festive meals, because in such a case, the joy of the festival is primary, and the joy of the Sheva Berakhot does not detract from it but rather reinforces it (SA 546:3).
The festive meals accompanying a brit mila or pidyon ha-ben can be held on Ḥol Ha-mo’ed (SA 546:4). The joy associated with these events is not great enough for us to be concerned about its overshadowing the joy of the festival.
An engagement party may be held during the festival, although some maintain that the refreshments must be light, meaning a full meal may not be served (Taz 546:2). One who is lenient and serves a meal has an opinion to rely upon (MB 546:2).