Viktor’s name day is November 11, the feast of Victor and Stephanida.
Churches that follow the Old (Julian) Calendar keep the same feast on November 24 of the civil calendar. See everything commemorated on November 11.
The saint behind the name

Victor and Stephanida
November 11, Martyr, Βίκτωρ
The holy martyr Victor was a Christian soldier who suffered for Christ in the second century, refusing to sacrifice to idols and enduring cruel tortures with unshaken faith. Stephanida, the young wife of another soldier, saw his endurance, confessed Christ openly, and was put to death with him.
Also celebrating on November 11
These names share the feast, many of them other forms of the same name:
Viktoras, Victor, Viktoria, Victoria
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.