During WWII, German forces occupied several islands in the Aegean Sea, including the island of Rhodes. With the Allied re-conquest of Greece, however, these German forces became isolated, with all resupply and mail service coming via air.
In December 1944, the Eastern Aegean Islands commander decided to print special feldpost stamps for troops on Rhodes, Leros, and Kos to use on mail sent home for Christmas. To accomplish this, he had 25,000 stamps supplied by the Italian director of posts overprinted "WEIHNACHTEN 1944" (Christmas 1944).
These stamps were distributed to German forces on 22 December 1944.
Despite the relative large printing (25,000), these stamps are fairly rare today.
They were printed in sheets of 50, but due to typesetting differences, 5 different types exist within each sheet.

And here are a few examples of the types:
Type I and Type II, columns 7-8 of the sheet
Type IV and Type III, columns 1-2 of the sheet
Type V and Type I, columns 5-6 of the sheet