Arethas’s name day is October 24, the feast of Arethas of Najran.
Churches that follow the Old (Julian) Calendar keep the same feast on November 6 of the civil calendar. See everything commemorated on October 24.
The saint behind the name

Arethas of Najran
October 24, Martyr, Αρέθας
St. Arethas was the chief of the Christians of Najran in southern Arabia in the sixth century. When a persecutor seized the city and demanded that its people renounce Christ, the aged Arethas strengthened his fellow Christians in the faith, and with a great multitude of men, women, and children he was put to death around the year 523.
Name days: Arethas
What to say on a name day
The Greek wish is Χρόνια Πολλά (Chronia Polla), “many years.” A call or a short message on the day is what people remember. See name day wishes you can send and what a name day is.