Nidas’s name day is February 18, the feast of Leo the Great.
Churches that follow the Old (Julian) Calendar keep the same feast on March 3 (a day earlier in leap years) of the civil calendar. See everything commemorated on February 18.
The saint behind the name

Leo the Great
February 18, Hierarch, Λέων ο Μέγας
St. Leo the Great was Pope of Rome in the fifth century, one of the great fathers of the undivided Church. His celebrated Tome, read at the Fourth Ecumenical Council at Chalcedon in 451, confessed with clarity that Christ is one Person in two natures, true God and true man, and the fathers of the council received it as the voice of Peter.
Also celebrating on February 18
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.