The context of this quote is that Pip has just come home from his first day at Miss Havisham's house. Mrs. Joe and Mr. Pumblechook want to know all about it. Pip feels incapable of explaining what it was like or and unwilling to expose to these two callous people how humiliated he felt at being called "common," so he makes up a tissue of lies, saying he and Estella and Miss Havisham sat in a black velvet lined coach in her room and played a game called flags. Later, Pip tells Joe what lies he told and about being made to feel inferior. Joe doles out comfort while telling Pip not to lie. Later, in his small bedroom, Pip thinks of Mrs. Joe and Joe sitting in the kitchen having supper and feels embarrassed at this. For the first time, it feels to him like common or low class behavior.
Pip then notes, as he stated it the quote above, that this had been a momentous day, and that we have all had days like this that stand out because they changed or influenced the course of our lives. Pip's life will not be the same again after visiting Miss Havisham's.
Placing this rumination of the adult Pip in context, we can already see the first change that has taken effect: Pip is now ashamed of his background and ashamed of Joe, a state of mind he will have to deal with through much of the novel.