Sofonias’s name day is December 3, the feast of Zephaniah the Prophet.
Churches that follow the Old (Julian) Calendar keep the same feast on December 16 of the civil calendar. See everything commemorated on December 3.
The saint behind the name

Zephaniah the Prophet
December 3, Prophet, Σοφονίας
The holy prophet Zephaniah, one of the twelve minor prophets, prophesied in the kingdom of Judah in the days of King Josiah, before the Babylonian captivity. He proclaimed the great day of the Lord, calling the people to seek righteousness and humility, and he foretold that a meek and lowly remnant would trust in the name of the Lord and rejoice in His salvation.
Name days: Sofonias
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.