What event signifies the beginning of the revolution on Animal Farm?

The uprising against Mr. Jones is indeed the pivotal event that signifies the start of the revolution on Animal Farm. This moment marks the culmination of the animals’ growing discontent with their neglectful and abusive human owner, Mr. Jones. It represents the animals uniting to take control of their fate and asserting their independence. In this act of rebellion, they successfully drive Mr. Jones and his men away, symbolizing their first taste of freedom and establishing the foundation for the new society they aim to create.

The other events listed occur later in the narrative. The pigs' secret meetings are a sign of their growing leadership and consolidation of power but do not directly spark the revolution. The building of the windmill is part of the animals' efforts to develop their new society after the revolution has already begun. Lastly, the adoption of the commandments reflects the animals’ attempt to formalize their new beliefs and governance but occurs after they have already taken over the farm.