When the Sunday morning meetings begin, they function as a celebration of the Rebellion and the overthrowing of Mr Jones. They also have a practical purpose in that they enable a discussion of any pressing issues and the preparation of the next week's work. They are not, however, wholly democratic: in fact, only the pigs ever put forward a resolution, as the reader learns in Chapter Three. The meetings always close with a rendition of Beasts of England and the rest of the day is set aside for recreation.
By Chapter Five, however, the Sunday morning meetings are changed considerably. After a discussion over the windmill, Napoleon uses unprecedented violence to oust Snowball from the farm and he puts an end to any future meetings. From this point on, all decisions are to be made in private by the pigs and without any input from the other animals.
Evidently, Napoleon feels that the Sunday morning meetings represent a threat to his authority. In this respect, the end of the Sunday morning meetings is symbolic of Napoleon's transformation from a leader into a dictator.