During the Christmas season, when Christmas displays and the public outcry against them get almost equal billing, the tiny postage stamp dares to push the envelope, so to speak. While some towns are battling over the use of red and green lights on city buildings, Nativity scenes in parks and what to call holiday evergreen trees, the tiny adhesive squares on billions of letters and packages this December will subtly remind postal workers and mail recipients about Christmas and other religious holidays. This year the Postal Service has issued more than 2.6 billion holiday stamps. The majority of them are...