Sisois’s name day is July 6, the feast of Sisoes the Great.
Churches that follow the Old (Julian) Calendar keep the same feast on July 19 of the civil calendar. See everything commemorated on July 6.
The saint behind the name

Sisoes the Great
July 6, Venerable, Σισώης
St. Sisoes the Great was one of the renowned desert fathers of Egypt, living in the fourth and fifth centuries. After years in Sketis he withdrew to the mountain where St. Anthony the Great had lived, and there spent long decades in solitude, prayer, and profound humility, receiving those who came for a word of salvation.
Name days: Sisois
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.