This Masechta discusses the laws pertaining to Chol Hamoed, the Intermediate Days of the Festivals. Why are these intermediate days referred to as חול המועד?
It is noteworthy that one literal translation of the word חול is sand. It is said (Shir HaShirim 7:2) מַה יָּפוּ פְעָמַיִךְ בַּנְּעָלִים בַּת נָדִיב, but your footsteps were so lovely when shod in pilgrim’s sandals, O daughter of nobles. The Medrash states that this verse refers to what is called נעילת החג. The Sfas Emes writes that the reason that the last day of the Festivals is referred to as נעילת החג, the locking of the festival, is because one has to safeguard the holiness of the festival.
The Gemara (Kiddushin 81a) states that the weakest part of the year is the festivals, and Rashi explains that men and women come to mingle on the festival , and this can lead to immoral activity. Based on these ideas we can suggest that the reason the intermediate days of the festivals are called חול המועד is to hint to the idea that just like the sand functions as a barrier from the sea, so too one must be extra vigilant on the festival not to come to sin. (This idea offers a hint to the expression of חול המועד, not necessarily related to the actual intermediate days of the festival.)
Daf Drush 