Some very conservative Christians do not use the conventional names for the days of the week. They do not want to invoke the names of false gods, so as I recall they call the days things like "first day", "second day" etc.
It might be fair to ask at this point: What type of "herd immunity" is being looked for? Immunity to an infection? Or "immunity" to certain ideas such as the existence of "inalienable rights" given by God?