In Harper Lee's To Kill a Mockingbird, the Chapter 9 description of Scout's Christmas experience at Finch's Landing is important because it gives us a look at the Finch family background and we learn more about Aunt Alexandra.
It's important to understand that the Finches' have a wealthy family background in order to better understand some of the conflicts Atticus enters into with his sister, Alexandra. We learn exactly how wealthy the Finches have been by reading Scout's description of Finch's Landing in Chapter 9. The actual farm stands on top of a "high bluff," which is another word for a cliff. There are "sixty-six steps" going down the cliff, and those steps end "in a jetty," which is another word for a pier or wharf. Waterside property is always worth a lot in value because waterside property is always the most fertile and attractive. Therefore, the fact that the farm stands on a cliff overlooking the river shows that the farm is worth a lot. Plus, it would have cost a lot of money to build the 66 steps leading down the cliff to the jetty; therefore, those steps also show just how much money Simon Finch, the Finches' founding ancestor, invested into the property.
Scout's description of the house further shows us how valuable the property is. The house is described as a very large, wealthy house characteristic of many houses found on Southern plantations in the days of slavery. It's described as a "two-storied white house with porches circling it upstairs and downstairs." It also has six upstairs bedrooms.
Since we know that the Finches are a very wealthy, white-Anglo Saxon family, we can better understand some of Aunt Alexandra's mannerisms and opinions. Since she considers herself to be from a family of good breeding, she looks down her nose at Scout being permitted to behave like a tomboy. In fact, we learn in this chapter that Aunt Alexandra dislikes Scout so much that, during Christmas dinner, she forces Scout to sit all by herself at the children's table even though the only other children in the family, Jem and Francis, have long been permitted to sit at the adults' table. As Scout phrases it, "Aunty had continued to isolate me long after Jem and Francis graduated to the big table."
In addition to ostracizing and ridiculing Scout because Aunt Alexandra herself had always been raised to be ladylike, Aunt Alexandra also disapproves of Atticus's involvement in Tom Robinson's case due to her background. Since the Finch family was a wealthy slave-holding family, Aunt Alexandra feels the Finches should continue to think that they are better than the African Americans and she rejects Atticus's view of treating all people with equal courtesy. According to Scout's cousin Francis, Aunt Alexandra thinks that Atticus being a "nigger-lover" is "ruinin' the family."
Hence, by contrasting Atticus and his children with the Finch family estate and the attitude of Aunt Alexandra in the Christmas scenes in Chapter 9, Harper Lee allows us to see just how strongly Atticus goes against the grain of his own society. Atticus's actions are being ridiculed by his own family members due to their wealthy, slave-holding background.