In Ch. 8 Pip visits Miss Havisham for the first time. At the end of that visit, Miss Havisham tells him "come again after six days." So accordingly, in Ch. 11 Pip visits her for the second time. This time Miss Havisham orders him into another room. Pip describes the room as a room from which "daylight was completely excluded, and it had an airless smell that was oppressive."
It doesn't take too long for Pip to discover the reason for the stink which pervaded that room. The most prominent object in that room was a long table covered with a table cloth on which it seemed as though a feast had been laid out. Pip after looking carefully realises that the "epergne or centrepiece ... was so heavily overhung with cobwebs that its form was quite indistinguishable." It had become rotten and seemed "like a black fungus" on which were crawling spiders.
Finally, the mystery is cleared up by Miss Havisham telling Pip who was not able to guess what it was that "IT'S A GREAT CAKE. A BRIDE-CAKE. MINE!" Before Pip is able to figure out what she meant by this, Miss Havisham catches hold of him and asks him to help her to walk around the room.
Later, in Ch.22 Herbert tells Pip how Miss Havisham was cheated on her wedding day by her fiancee: " the marriage day was fixed, the wedding dresses were bought, the wedding tour was planned out, the wedding guests were invited.The day came, but not the bridegroom." Miss Havisham obviously went into a state of shock from which she has still not recovered and she has left the wedding cake on the table to continue to rot till today.
During his first visit to Miss Havisham's in Ch.8 Pip describes how she was dressed : "she was dressed in rich materials - satins, and lace and silks - all of white. Her shoes were white. and she had a long white veil dependent from her hair, and she had bridal flowers in her hair, but her hair was white.
The dress which Miss Havisham is wearing now is the same wedding dress which she wore in preparation for her wedding which did not take place, and the rotten mass which Pip sees in Ch. 11 is the wedding cake which was never used.