Some hints for newcomers, who are not so familiar with the german shop-closing habits:
This year, the two days of Christmas will be on a Thursday and a Friday. All shops will be closed on these days, with a few exceptions in the Main Railway Station and at the Airport.
The 24th of December, a Wednesday, is no official public holiday in Germany. But strangely enough, this evening is the most important time for a family get-together. This is why the entire retail sector will come to a standstill in the early afternoon. All supermarkets, drugstores, department stores and shopping centres can be expected to close at around 2 or 3 pm, at latest. Even most of those at the Main Station and Airport. Only some kiosks and gas stations may offer a very limited choice. (Please note, that furthermore, nearly all eating places will be closed that evening too. Even McD, BK, KFC etc.)
Saturday the 27th will be a normal business day for retailers. Only, that most stores will get no or little supplies or will not have the time to put that on the shelves. So many items can be expected to be sold out. Not to mention that shops will be heavily crowded.
My advice is, to get all things as early as possible. Maybe as early as this week.
Tidings of comfort and joy. ;)