I found this article at Snopes.com. The United States Postal Service REMEMBERS and HONORS the EID MUSLIM holiday season with a commemorative first class holiday EID postage stamp. The stamp was introduced by the United States Postal Service (at the then-current 34-cent rate) as part of their Holiday Celebrations Series on 1 September 2001, just ten days before the September 11 terrorist attacks on New York and Washington. It was reissued by the USPS in October 2002 at the new 37-cent rate.

The EID stamp is often mischaracterized as a "Christmas stamp" even though it has nothing to do with Christmas other than that it is part of a series of U.S. postage stamps commemorating several diverse celebrations (Christmas, Hanukkah, Kwanzaa, Thanksgiving) which sometimes occur at roughly the same time of year........The statement "[Muslims] don't even believe in Christ and they're getting their own Christmas stamp!" is nonsensical, akin to protesting Hanukkah stamps because "Jews don't even believe in Christ but they have their own Christmas stamp."

I'm not sure that I would want to have this "Christmas Stamp" on my Christmas cards this year. They may be more appropriate on the holidays that they signify. Read the rest of the story