Old Major dies three nights after giving his speech. It is presumed that he dies due to old age. He had alluded to this in his speech. His death is significant for a couple of reasons. Given the fact that he dies only three nights after giving his speech, he does not organize any sort of political action or configuration to help bring his dream into reality. He dies with his vision in a theoretical state. Due to this, the pigs assert leadership. This is critical because since Old Major's vision guides them, there is very little leadership in his absence.
Such a vacuum enables Snowball, Napoleon and Squealer to control the affairs of the farm. They are the ones to organize the revolution. They are able to do so because Old Major died so quickly after his speech. There is no organization that followed his speech, so the pigs create and construct the organization. Old Major's death represents the difference between declaring independence and freedom and actually achieving it. In this, his death three days after giving his speech becomes highly significant.
Old Major passes away three nights after. Even though Old Major dies they still sing the song beats of england.
Since Old Major passed away very quickly, the farm had no set of rules which was supposed to be followed. Instead, they randomly made up their own set of rules which had to be followed.