At the start of this chapter, Oceania is at war with Eurasia. It is Hate Week and there have been six days of build up, all focusing on hatred for Eurasia. At the peak moment of Hate Week, however, there is a change -- Oceania is now at war with Eastasia and Eurasia is their ally.
The government treats the change about as you might expect -- they pretend it is not a change. Remember, this is a government that does not allow people to remember the facts about their past. Everything the government says is true and it can never be wrong. Therefore, the government cannot admit things have changed. Instead, they go on with Hate Week as if nothing had happened.
Here is a quote to show this:
There was, of course, no admission that any change had taken place. Merely it became known, with extreme suddenness and everywhere at once, that Eastasia and not Eurasia was the enemy.
Of course, the posters and everything are wrong -- they have the wrong names and faces. Therefore, the government puts out the idea that Goldstein and his people had sabotaged things but making it seem as if Oceania was at war with Eurasia.
The banners and posters with which the square was decorated were all wrong! Quite half of them had the wrong faces on them. It was sabotage! The agents of Goldstein had been at work! There was a riotous interlude while posters were ripped from the walls, banners torn to shreds and trampled underfoot.