That was a memorable day to me, for it made great changes in me. But, it is the same with any life. Imagine one selected day struck out of it, and think how different its course would have been. Pause you who read this, and think for a moment of the long chain of iron or gold, of thorns or flowers, that would have bound you, but for the formation of the first link on one memorable day.
( Volume I / chapter ix )
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.