What does he write about in his diary?

Winston writes, "April 4th, 1984," and then realizes he is not even certain of the year, as it is impossible to tell if the information the Party disseminates is truly accurate anymore.

Winston begins writing about a violent war film with vivid death scenes. He then remembers an event from earlier in the day that inspired him to begin the diary. It occurred at about eleven hundred that morning (time is kept in the twenty-four hour method) during the Two Minutes Hate, a daily propaganda presentation given to groups at their places of work praising Big Brother, Oceania and the Party, and denouncing Emmanuel Goldstein, the figurehead of capitalism and the Party's number one enemy, and Oceania's current enemy of war. While surrounded by fellow Party members caught up in the fervor of denouncing enemies to the Party, literally screaming and throwing things at the screen and praising Big Brother and Oceania, Winston took note of those around him. He observed the dark-haired girl he had often seen in the Ministry who he hated based purely on her apparent worship of the Party, and also a man named O'Brien, an Inner Party member whom he also often saw in the Ministry of Truth. He and O'Brien made eye contact, and immediately Winston felt as though they were both thinking the same things, realizing that O'Brien also found this practice and the Party's propaganda disgusting. O'Brien, he suddenly understood, also yearned for individual freedoms. Bolstered by what he perceived to be nonverbal support of his anti-Party feelings, Winston resolved to begin his diary that day.

While remembering this event, Winston finds he has unknowingly written, DOWN WITH BIG BROTHER over and over in his diary.