The poet promised to remember Jerusalem forever. He called down imprecations on himself if he ever were to forget the city that had been the scene of so much joyful worship in the past. The hand and tongue stand for all action and speech (by synecdoche). One reason the Israelites loved Jerusalem so much, was that it was the site of their annual festivals-that were mainly joyous occasions of praise and fellowship (cf. Lamentations 1-2). [source][source][source]