Early in the novel, passage of time is used to show how the animals are adapting to their new lives as masters of the farm. They work hard, secure in the knowledge that the humans will not return to subjugate them, and feel satisfied in their labor:
All through that summer the work of the farm went like clockwork. The animals were happy as they had never conceived it possible to be. Every mouthful of food was an acute positive pleasure, now that it was truly their own food, produced by themselves and for themselves, not doled out to them by a grudging master.
(Orwell, Animal Farm, george-orwell.org)
The passing summer acts as a metaphor for the time of joy directly following the revolution. It is too soon to recognize the dangers of the pigs taking a supervisory role while the other animals work. Because the summer goes so well, the animals are complacent and content, not thinking about future ramifications. For example, the pigs continue to learn human skills while the other animals are kept performing their typical tasks; one "class" is educated, while the other is kept ignorant. When the harvest is done and the winter sets in, after a failed attack on the farm by humans, the animals start to show discontent with their lives, starting with Mollie, who wants to be taken care of instead of working with the others.