But celebrating their religious observances in home and church is a clause with celebrating as its verb, not a noun phrase with observances as its head. The word celebrating cannot possible [sic] be an attributive modifier with observances, because their, a genitive pronoun, follows it; that cannot be anything but a determiner, and determiners precede attributive adjectives. (Genitive pronouns cannot serve as attributive modifiers: phrases like *the my house or *an our cat are utterly ungrammatical.)
I also love that I found a typo. In Language Log.