Poimenas’s name day is August 27, the feast of Poimenas the Great.
Churches that follow the Old (Julian) Calendar keep the same feast on September 9 of the civil calendar. See everything commemorated on August 27.
The saint behind the name

Poimenas the Great
August 27, Venerable, Ποιμένας
St. Poimen the Great was one of the most renowned of the desert fathers of Egypt, living in the fourth and fifth centuries. With his brothers he took up the monastic life in Sketis, and became a discerning elder to whom monks came from afar for a word of guidance.
Name days: Poimenas
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.