The fairest and most effective counterclaim is:
Halloween is a holiday rooted in the rituals of ancient religions, some of which are practiced today, and their followers always observe it on October 31.
This counterclaim directly addresses the cultural and historical significance of Halloween being on October 31, suggesting that moving the holiday could undermine its traditional values and practices. It provides a strong rationale for keeping the holiday on the date it has been celebrated for many years, making it a relevant and compelling argument against the proposed change.