Read the first part of the article: Birthdays and Namesdays celebrations in Poland - with lots of information about selected namesdays and their patrons. Read also about The most Common Baby names in Poland .
Poles celebrate namesdays (imieniny) more than the birthdays (urodziny), but not every popular first name has its saint thus the day devoted to this saint in the calendar, therefore Polish calendar has to be modified to include also these popular names. For instance female names: Iwona (Yvonne) or Grazyna became popular during the last 50-100 years but there is no any saint figure or patron to these names.
My first name Jagoda (which means just "blueberry") was just recently included in the calendar on July 2nd. There is a justification why this name was included in the calendar just at this date, you can read more about it in the article: Polish Customs, Tradition and Folklore by Sophie Hodorowicz-Knab. When I was a child, my name was not in calendar so I celebrated birthday every year instead of the namesday as the majority of Poles do. Since my birthday is just a week before Christmas this was not really convenient day for a party and for receiving of many extra souvenirs.
Here are the links to the Polish calendars available on the internet:
http://www.kramsk.pl/rozne/imieniny.html - search by name
http://www.masterpage.com.pl/imieniny.html - search by day in a calendar (the names are in a genitive form)
http://www.slavinja.republika.pl/tekst3.htm - search in a calendar which contain only old Slavic names, some of them are very rarely used.
Some namesdays are infamous for their celebrations, for instance St. Stanislaus day. It is better not to drive during the evening of Stanislaus namesday because there are too many drunk drivers on the streets! The police tries to be especially alert and controlling during the evenings or the weekends of the especially popular namesdays.
Poles celebrate their birthdays also but not to the extend they celebrate their namesdays. Some birthdays are more important than the other - for instance when you are 18, 21, 21, 30, 40 and of course a 100! If you are lucky enough to survive!
How the namesdays and birtdays are celebrated?
Poles usually invite people for their namesday or birtday party, but even if you are not invited you can call with the birthday wishes or drop by the house of a person who has a birthday. Poles also celebrate namesdays at their work place, they invite their co-workers to eat a cake with them, but alcohol drinking at work is not really allowed.
If you are invited for birthday or namesday to somebody's home you should bring flowers (read more about flowers in the article Polish Flowers - Funerals, All Saints Day, Christmas and other Occasions - Birthdays, Women Day), you can also bring a good alcohol (usually for an adult man) and a simple gift (for instance a bottle of perfumes or a good book).
The host who is celebrating birthday or namesday (in Polish: solenizant) would receive you with a cake - this is usually a round cream cake called in Polish: tort . It can have candles on. The cake is accompanied by coffee and tea and some good alcohol (especially if this is a male birthday). Very often you would receive a good supper or a dinner before a cake is served.
If you cannot attend the birthday or namesday because you are far away it is customary to send a postcard with good wishes. But if you live in the same town as a person celebrating the birthday or namesday - sending a card is not a norm. I mean, people who live in the same town do not send postcards to themselves whether this is a birthday, Christmas or... whatever occassion, the telephone call or a visit is a much better solution. I still remember how I was amazed to receive some Christmas cards from people who were living in the same town as I did after moving to the US.
Poles have some special songs which are sung during birthdays before the toast.
The most famous song is called "The Hundred Years", in Polish: "Sto Lat".
Below is its refrain:
Sto lat, sto lat, niech zyje, zyje nam!
Jeszcze raz, jeszcze raz, niech zyje, zyje nam!
For a hundred years, for a hundred years,
May he/she live for us.
I also recommend Polish First Names Author: Knab, Sophie Hodorowicz