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Chol Hamoed is the name for the days during Succot and Pesach that fall in between the Yomtovim at the beginning and the end. In Israel there are six days of Chol Hamoed in Succot; outside Israel, there are five.
The name Chol Hamoed contains within itself a tension. Chol means secular or profane - the opposite of kodesh which is holy. Moed means a meeting, or a special time - thus moedim is the collective name given to the Jewish holidays as a whole. So Chol Hamoed, means the secular time within the holidays.
Nevertheless, Chol Hamoed is a lot more moed and less chol than most people realise. The days are supposed to be a time of celebration, almost like Yomtov. Halachically, the Yomtov restrictions on work apply to Chol Hamoed, with two major, important relaxations; one may do work for a living if the consequences of not doing it would be substantial monetary loss, and one may do work which is necessary for the enjoyment of the holiday - including activities such as shopping, cooking and driving.