Winston mentions the Golden Country dream in his real-life encounter with Julia in Book 2, Chapter 2. But the dream itself is described earlier in the book, in Book 1, Chapter 3. In it, Winston finds himself in a bucolic country setting, where he witnesses a beautiful woman with dark hair (like Julia) walking through a meadow towards him. She removes her clothes with a single gesture, and Winston admires her, not so much for her obvious physical beauty, but for the nonchalance with which she casts her clothes aside. He understands this gesture to be a rejection of everything the Party stands for:
With its grace and carelessness it seemed to annihilate a whole culture, a whole system of thought, as though Big Brother and the Party and the Thought Police could all be swept into nothingness by a single splendid movement of the arm. That too was a gesture belonging to the ancient time. Winston woke up with the word ‘Shakespeare’ on his lips.
The fact that he mentions Shakespeare and the "ancient time" suggests that the pastoral setting, the girl, and indeed eroticism itself are understood by Winston to belong to an idealized past, one which the Party has attempted to efface from the collective memory of its members. Winston very much longs for that past, and feels that he has it when he meets Julia in the flesh in Chapter Three of Book Three. But their relationship turns out to be as fleeting as Winston's dreams, as they are eventually turned in and tortured by the Party.