"Wintermas" is supposed to the the secular substitute to Christmas. However, the -mas comes from Mass, a religious term, and while it would be winter in the northern hemisphere, it would be summer in the southern hemisphere.
Where are the other religious groups? Where are the Jews, the Muslims, the Hindus, the various pagans, and who knows what else?
Reflective of the author's complete inability to understand anyone actually believes in any (version of) god than his specific variety of Christian one, and isn't just non-Christian out of some need to rebel. It came up in Left Behind as well, that everyone not Real True Christian was happy to abandon their religion as soon as the evil world government told them to.