Sergei’s name day is September 25, the feast of Sergius of Radonezh.
Churches that follow the Old (Julian) Calendar keep the same feast on October 8 of the civil calendar. See everything commemorated on September 25.
The saint behind the name
Sergius of Radonezh
September 25, Venerable, Σέργιος του Ραντονέζ
St. Sergius was born near Rostov in the early fourteenth century and, as a young man, withdrew with his brother into the forest of Radonezh, where he built a church of the Holy Trinity. The brotherhood that gathered around him became the Trinity Lavra, the spiritual heart of Russia, and his disciples founded monasteries across the land.
Also celebrating on September 25
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.