It should read, "Everyone celebrates religion and holy days in his own individual way, but, because of P.C. we now have the awkward, "their" upsetting the flow of the words.
Most municipalities have sign ordinances, many have been restricting what may appear on the sign; if that were the case here, there would be a reason for the outrage.
You missed my point... conveniently. This article is about mixing religion and holidays... and at the same time holiday comes from "holy day".