Let us（ ）the festival together.

B
let是情态动词,
情态动词后面必须加动词原形,类似的还有
①只做情态动词：must,can(could),may(might),ought to
②可做情态动词又可做实义动词：need,dare
③可做情态动词又可做助动词：shall(should),will(would)
④具有情态动词特征：have(had,has) to,used to