欢聚一堂 have a joyous gathering (with); a happy gathering [reunion]; be gathered happily in the same hall; be together on a happy occasion; enjoy a nice meeting together; get together joyously; happily gather under the same roof
济济一堂 a gathering of many people; (a big audience) filled the hall.; a great assemblage of people in a hall; a large gathering in an auditorium; gather together under the same roof; the house is packed with guests.: 老、中、青济济一堂, 共同学习, 相互激励。 gathered together, the old, the middle-aged and the young studied side by side and encouraged each other