JDM was a self-declared agnostic, and I can only guess that Christmas wasn't a big deal in the MacDonald house. "Dead on Christmas Street" is a fine mystery story, but it really has little to do with the holiday. It's a straightforward tale of crime solving that happens to take place during the Christmas season, and I'm tempted to surmise that the setting was dictated by an editor, who may have wanted a Christmas theme for his December 20th issue. The story does not depend on the season and it would have worked just as well at any other time of the year.
The primary witness in an upcoming trial has been pushed out of a high rise office window a few days before Christmas, and without her the district attorney has no case. Dan Fowler is the Assistant DA assigned to the trial, and he knows that the three teenagers accused of the robbery she witnessed couldn't have been responsible, since they are currently sitting in jail. One of them, however, is the younger brother of local crime boss Vince Servius, and Dan immediately suspects he may have been involved. When Servius provides an alibi and convinces him that he doesn't much care for his younger brother, Dan and his fiancé Jane (who also works in the DA's office) begin checking into the background of the dead witness. They uncover a not very likeable woman with a lifestyle far beyond the means of an ordinary secretary.
"Dead on Christmas Street" seems an odd inclusion for the Christmas issue of a mainstream periodical, but MacDonald had already published several stories there, including the serialized "My Brother's Widow," which later became the novel Area of Suspicion. Nearly 20 years later "Dead on Christmas Street" was reprinted in the Christmas issue of a more appropriate monthly, Ellery Queen's Mystery Magazine. It has been anthologized at least twice, originally in 1990's Mystery for Christmas and Other Stories: From Ellery Queen's Mystery Magazine and Alfred Hitchcock's Mystery Magazine and again in 2002 in the collection titled Murder Most Merry. Used copies of both books are very easy to locate.