What's a name day?
A name day is a fine old religious tradition, now mostly observed in the Orthodox Christian countries of Eastern Europe and Catholic countries in Latin and South America. In these traditions, everyone is named after a saint, each of whom has a feast day that is the person's name day. So, for example, if you are Tatiana, your name day is Jan. 25, the feast day of St. Tatiana.
In very religious homes, a name day will begin in church and end with a party. In extremely religious homes, the family will celebrate their name days and not their birthdays, with gifts, cards, and feasts.
In less religious homes it is celebrated like a second birthday.
If you're invited to a name day celebration, don't worry about appearing suitably religious — it will be pretty obvious if you aren't. Bring flowers and something sweet. Follow the lead of the folks around you. Have fun but don't become drunk or rowdy. Except on Tatiana's Day, where a bit of rowdiness is sanctioned from on high.