Having grown up in a Calvinist country I never knew anybody who celebrated their own or another's onomastico. Then, a non-practicing Orthodox from Eastern Europe married into the family and his onomastico day was more important to him than his own birthday.
An onomastico day is your saint's day. At baptism everybody gets a Christian name. On Palm Sunday in the year of salvation 1979 I received the Sacrament of Baptism and was given the name John. So, December 27th is my onomastico day, the feast of St. John.
Try to celebrate your name day. Have a devotion to your saint. Read the lives of the saints. Follow the Church's liturgical calendar. Enrich your life and that of your family.