The Entmoot was a gathering and meeting of Ents. An Entmoot didn't happen very often.
In Fangorn Forest, these moots were always held in Derndingle, a bowl-like valley far south of Treebeard's home. Since the Ents' language was so descriptive and extensively slow, an Entmoot would endure unbearably long.
The Entmoot held in The Lord of the Rings was held from February 30 to March 2, 3019, of which the subject was the decision whether the Ents should go to war against Saruman. It lasted from noon on the first day till afternoon on the third day, and by Ents, it was considered quick for an Entmoot. 
In Dutch, an Entmoot is called Entmoet, which is similar to the Dutch word Ontmoet (to meet).