Pistis’s name day is September 17, the feast of Sophia and Faith, Hope, Love.
Churches that follow the Old (Julian) Calendar keep the same feast on September 30 of the civil calendar. See everything commemorated on September 17.
The saint behind the name

Sophia and Faith, Hope, Love
September 17, Holy Woman, Σοφία, Πίστις, Ελπίς, Αγάπη
St. Sophia was a pious widow of Rome in the second century who raised her three young daughters, Faith, Hope, and Love, in devotion to Christ. During the persecution under the Emperor Hadrian the three girls confessed the faith one after another and were put to death before their mother's eyes, and Sophia, having buried them, soon followed them in death from her grief.
Name days: Sophia, Sofia, Sofi, Elpida, Agapi, Pistis, Faith, Hope, Love
Also celebrating on September 17
These names share the feast, many of them other forms of the same name:
Sophia, Sofia, Sofi, Elpida, Agapi, Faith, Hope, Love
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.