In Part Two of this book, Julia and Winston meet a number of times in the course of their affair. I am going to assume that you are asking about the first time they meet and have sex with each other.
This rendevous is set up for a secluded area out in the country. We are not told where it is -- like what town it is near or anything. Julia has been there before, but Winston has not, except in his dreams. She tells him exactly how to get there, what train to take, which way to turn going out of the station, how far to go, etc.