Μὴ οὖν τις ὑμᾶς κρινέτω ἐν βρώσει καὶ ἐν πόσει ἢ ἐν μέρει ἑορτῆς ἢ νεομηνίας ἢ σαββάτων, NA27
The list of items in which not to allow yourselves to be judged is delimited by ἐν: «ἐν βρώσει...ἐν πόσει...ἐν μέρει ἑορτῆς ἢ...»
I can’t clarify in my mind what the significance μέρει brings to the text. ἐν μέρει is nearly universally translated as “in regard to,” but isn’t “regard to” implied in the other instances of ἐν? What is distinct and different in the third ἐν that it requires μέρει, and what does μέρει add that isn’t implied by the other instances of ἐν?