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.

The passage is clearly about what hindsight reaveals about a person, as Pip is reflecting on this point in his life and his development dure to the choices he made.

It is also reminiscent of Jacob Marley in Dickens' ' A Christmas Carol' explaining to Scrooge that he is bound in death by the chans he forged in life with his poor treatment of others-

`I wear the chain I forged in life,' replied the Ghost.
`I made it link by link, and yard by yard; I girded
it on of my own free will, and of my own free will I
wore it. Is its pattern strange to you?'

This passage from Chapter IX reminds me of Robert Frost's poem, "The Road Not Taken"; for, the speaker of this poem reflects that he has chosen another path and "that has made all the difference."  This "first link" of Pip's--his exposure to another level of society and its superior attitudes--has altered his perception of himself and his surrounding.  No longer satisfied with his life, ashamed of being "common," Pip desires to become a gentleman so that he will be the equal of anyone, but in so doing, he clearly changes the course of his life. For, he abandons for a long time the true values of life: frienship, unselfish love, and integrity.

In addition to the above posts, I would suggest that Pip recognizes that life is filled with both the positive (the gold and the flowers) and the negative (the thorns and the iron). As pointed out, given that adult Pip is retelling his story, he can look back on his childhood with the critical thinking skills of an adult.

Pip is commenting as an adult on his childhood experiences.  As the adult narrator, he has a unique perspective on events.  He can look back and see which ones are most significant in what happened to him later.  Arguably, the meeting with Miss Havisham completely changed the course of his life, even though it did not seem to at the time.

Basically, what Pip is saying here is that this day is going to change his life completely.  This is the day where Pip has first gone to Miss Havisham's house.  He sees this, looking back, as the day that started him off towards being a gentleman.