Harry and Hermione — Their Sixth Year is the second installment of three books. Its prequel is Harry and Hermione Start their Sixth Year. It covers the time from autumn until after the yule break. DrT writes about it:
Part 2 of my Harry and Hermione series, covering late October through early January. Voldemort crashes a party, house elves abound, Hermione meets the Dursleys, and of course the Halloween season isn’t as much fun as it should be.
They spend the christmas at Grimauld Place. Snape and Malfoy get gutted on christmas eve because they won’t follow a halfblood. There is also an interesting inheritage.
It can be read online on Schnoogle.
I also made a PDF out of its 10 chapters.