The whisky incident refers to the first time the pigs sample alcohol.  Orwell describes them in a sort-of blundering fashion; because the large amounts of alcohol they consumed has made them ill, the pigs begin to think they, and most importantly Comrade Napoleon, are dying.  Orwell may have used humor here to give the reader a chance to laugh at the pigs, to show us that the pigs are not infallible even though they have taken control over the farm and consider themselves the "brain workers."

A key significance to this moment is that the next day the pigs, of course, change the commandment which originally banned the drinking of alcohol by animals so that it included the phrase "to excess" - another blatantly corrupt use of the pigs' power.

Following the Battle of the Windmill, the pigs of Animal Farm found a case of whiskey in the cellar of the farmhouse. Although the Fifth Commandment declared that the consumption of alcohol was forbidden, the pigs decided to celebrate the victory. After a round of singing, Napoleon appeared in a bowler hat and then galloped around the yard. The next day, the pigs slept late, and it was later announced that Napoleon was dying. But he was only hung over, and he soon recovered. The author no doubt created a humorous tone because the visualization of drunken pigs dressed in human clothing is quite funny. He also used it to show how the pigs were further adopting human traits.