Thadeos’s name day is August 21, the feast of Thaddaeus the Apostle.
Churches that follow the Old (Julian) Calendar keep the same feast on September 3 of the civil calendar. See everything commemorated on August 21.
The saint behind the name

Thaddaeus the Apostle
August 21, Apostle, Θαδδαίος
St. Thaddaeus, numbered among the Seventy Apostles, was a Jew of Edessa who came to Jerusalem, heard the preaching of John the Forerunner, and became a disciple of the Lord. According to the ancient tradition of the Church, after Pentecost he was sent to Edessa, where he healed the ruler Abgar, preached the Gospel, and established the Church in that city.
Name days: Thadeos
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.