Eustratios’s name day is December 13, the feast of Eustratius and companions.
Churches that follow the Old (Julian) Calendar keep the same feast on December 26 of the civil calendar. See everything commemorated on December 13.
The saint behind the name

Eustratius and companions
December 13, Martyr, Ευστράτιος
The holy Five Martyrs Eustratius, Auxentius, Eugene, Mardarius, and Orestes suffered for Christ in Armenia during the persecution under Diocletian at the turn of the fourth century. Eustratius, a distinguished imperial officer, confessed Christ openly and endured many torments together with his companions, each of whom sealed his faith with his blood.
Name days: Eustratios, Efstratios, Stratos
Also celebrating on December 13
These names share the feast, many of them other forms of the same name:
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.