Seven is the most mentioned number in Harry Potter, no surprise there. (Seven books, seven years, seven [sort-of] Horcruxes, et cetera, et cetera, et cetera) An interesting "seven" are the seven tasks Harry must go through in Harry Potter and the Philosopher's Stone, which, in some ways, can mirror what has happened in the books.
Yes, I will explain, dear reader. Of course!
Task One
Fluffy.
Philosopher's Stone Connection
Fluffy. (Fluffy is in Philosopher's Stone, lame, but it starts us off.)
Task Two
The Devil's Snare.
Chamber of Secrets Connection
In Chamber of Secrets, Harry must travel through an underground chamber, slimy and dangerous, just as is in the second task. Also, plants (Devil's Snare and the mandrake) are included in both.
Task Three
The keys.
Prisoner of Azkaban Connection
Prisoner of Azkaban relates directly to flight, escape, and elusion, and those three are major themes in the third task. Also, capture plays a large role, as well as searching for the right fit to the lock. In this case, the right fit was not Sirius, but Peter.
Task Four
The troll.
Goblet of Fire Connection
While this may not be a task, it pertains directly to the fourth book. In Goblet of Fire, Harry is aided tremendously by someone who does not appear as they seem. In the fourth book, it is Barty Crouch Jr., posing as Moody, who does much of Harry's work for him. In the task, it is Quirrel, as Voldemort's minion, who does much of Harry's work for him.
Task Five
The chess set.
Order of the Phoenix Connection
In the fifth volume of Harry Potter, much of the plot has to do with sacrifice and deciet, as is in chess. While Ron sacrifices himself to the "Queen", Harry is able to escape in the task. In the book, Sirius sacrifices himself to a woman who can be so called the "Queen", Bellatrix Lestrange, so that Harry can escape safely.
Task Six
The potion riddle.
Half-Blood Prince Connection
Snape! The potion's master.
Task Seven
Voldemort!
Deathly Hallows Connection
Voldemort!
