I see no problem with governmental endorsement of even blatantly religious holidays if they are celebrated by the vast majority of citizens, those who belong to the religion in question, or not, as long as this holiday isn't inherently offensive or likely to cause harm. (IE, "kill the infidel day", with prizes for the best infidel hunter )If Christmas lights *were* religious, then it is irrelevant to whether or not a high percentage of people celebrated Christmas. Even if you don't argue that the US is not secular, you must argue that it is pluralistic, as freedom of the expression of religion is fundamental to US society. Then it is a matter of including other religious expressions as well to not endorse one religion. The argument then is the same of the inclusion of the 10 commandments in government buildings; if the 10 commandments were allowed, then other religious laws must be allowed as well.
Of course, I don't have a problem with putting up Menorah's for Hanukkah, or crescents for Ramadan, if a substantial portion of the population desires it.