One activity that is forbidden on Hol HaMoed is a happy one. People are not allowed to get married on Hol HaMoed, because, as the Mishna teaches on 8b, ein me’arvin simha be-simha – we are not supposed to mix one celebratory event with another.
The source for this concept that is suggested by the Gemara is the story of King Solomon’s consecration of the Temple, which took two weeks and ended prior to the Sukkot holiday (see I 8:65). This is understood by the Gemara to indicate that Shlomo felt that he could not allow the festivities connected with the Temple to impinge on the festivities of Sukkot.
It is interesting to note that although King Solomon was not willing to celebrate on Sukkot, the celebrations did take place on Yom Kippur. The Gemara records that these celebrations included food and drink, because ein simha b’lo akhila u’shetiya (there is no joy without eating and drinking). This conclusion may be based on the fact that we find that establishing an altar includes the bringing of sacrifices and eating them (see 12:7, 27:7). Rabbi Parnakh quotes Rabbi Yohanan as teaching that the Jewish people feared for their lives, given that they did not fast as required on Yom Kippur. Nevertheless, the Gemara concludes, a heavenly voice came from the heavens promising all of the participants that they would have a place in the World-to-Come. This is based on I 8:66, which teaches how the people all returned home joyous and happy.
The Gemara adds an explanation to the rule forbidding marriages on Yom Tov – that the preparations for the event are so involved that they will detract from the participants’ ability to fully enjoy the holiday. Thus, while Tosafot ask whether other types of celebrations should be forbidden on the holiday because ein me’arvin simha be-simha, the Ritva argues that only weddings, whose meals are so involved as to keep someone from being able to properly celebrate, cannot take place on Hol HaMoed; other festivities, however, would be permitted.