The Entmoot was a slow council that took place among the Ents. The slow speech and weighing of decisions usually took a longer time than what most races liked. It is unknown how many Entmoots took place across the centuries, but at least two would have had to occur. The first was in the Second Age where Treebeard states to Merry and Pippin that he has not seen them (Ents and Huorns) roused since the destruction of their forests.
The other known Entmoot took place when Merry and Pippin were in Fangorn Forest, which lasted for days--from February 30 to March 2 T.A. 3019 to be exact, and the decision was made to march on Isengard.