Onomastico is the name-day event in which Catholics celebrates all persons whose name coincides with the one of the saint of the day in the religious calendar.
Most calendars in Italy resemble the one pictured below; under the name of the week, it’s featured the name of a Saint on each day of the year.
Some name-days are more known than others, like December 26: St Stephen, or March 19: St. Joseph, etc.
In short, on the day of the saint with the name like yours, you get celebrated also.
For example, March 17 is Saint Patrick´s day. If your name is Patrick, people would congratulate you on that day and even give you presents, like for a birthday…
Well, my name is Marina and June 18th is Saint Marina, hence the title of this post [happy onomastic to me]. My mom called me from Italy this morning to wish me a good onomastic.