筵 and 席 were both ancient seating mats - before tables and chairs, banquets were eaten cross-legged on the floor. 筵 was the larger ground-mat, 席 the smaller seat-mat placed on top. The compound now means the banquet itself.
culture
The proverb 天下没有不散的筵席 ('no banquet lasts forever') is a common consolation for partings - friendships, jobs, even good times all eventually end.
