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Christmas Eve and New Years Eve are both not official holidays in Germany. It might be, that a lot of shops close earlier on these days than usual, but generally, you will nevertheless have enough options to buy food. And at last shops at airports, main stations and similar locations are open 365 days a year, so you want have to starve.
Also hostels and hotels are generally available 365 days a year in Germany. However, Christmas Eve is on a short notice and New Years Eve is in great demand, so you should check as soon as possible if there is an accommodation that suites your need.