Polish national holidays (bank holidays) and other celebrations.

December 31st and January 1st - New Year's Eve (Sylwester) and New Year's Day (Nowy Rok)
In Poland people joyfully celebrate New Year's Eve (Dec 31st) on streets, market squares and private parties at homes or in restaurants/pubs. December 31st isn't a day off from work so all the events start after 8 PM. New Year's Day is a day of leisure when people rest after all night long partying and drinking. All supermarkets are closed, only some small family-owned shops are open. Restaurants are usually open but frequently after 3 PM. Museums are usually closed on New Year's Day. 

January 6th - Epiphany or Three Kings (Trzech Króli)
It's a Catholic holiday with religious parades organized in many Polish cities, the biggest in Krakow and Warsaw. All supermarkets are closed, only some small family-owned shops are open. Restaurants are usually open but frequently after the parades are over.  Museums are usually closed. 

March or April - Easter celebrations:

Easter Thursday and Friday
The Paschal Triduum before Easter isn't a bank holiday in Poland so people go to work. On Maundy Thursday and Good Friday all shops are open and so are restaurants and pubs but in smaller towns they might be a little bit empty because it's the time of great preparations at homes as well as the time of prayers and contemplation.

Easter Saturday
It's the last day of the Paschal Triduum. On Easter Saturday people attend short ceremonies in churches. They all take small backets to bless the food, especially eggs, bread and salt, and these ceremonies mark the official end of the Lent. On Easter Saturday families usually paint hard-boiled eggs with different colors and patterns, those eggs are then called "pisanki". On Saturday supermarkets are usually open only to early afternoon, restaurants and pubs in big cities are usually normally open.

Easter Sunday
It's a religious and public holiday so all supermarkets are closed, as well as museums and galleries. Restaurants are open but some of them may be reserved for weddings which are frequently organized during Easter. People stay at homes and eat traditional Easter breakfast. Then in the afternoon they frequently go for walks.  Museums are usually closed. 

Easter Monday
It's called "Śmigus Dyngus" or "Lany Poniedziałek", wet Monday. People pour each other with water, sometimes not only at homes but also on streets. It's also a bank holiday like Easter Sunday.

May 1st - International Workers' Day or May Day (Święto Pracy)
All supermarkets are closed, only some small family-owned shops are open. Restaurants and pubs work as normal. Some museums may be closed. 

May 3rd - May-3rd-Constitution Day (Święto Konstytucji 3 Maja)
All supermarkets are closed, only some small family-owned shops are open. Restaurants and pubs work as normal. People frequently make the so-called long weekend between the May 1st and 3rd by taking a day off on May 2nd. It's the time of Polish "great migration" from cities to the countryside, for example many Warsaw or Krakow citizens head for Zakopane.

Thursday in May or June (60 days after Easter) - The Feast of Corpus Christi
It's a religious and public holiday so all supermarkets are closed but restaurants and pubs are usually open. There're religious processions all over the country, some of them are very big (e.g. Krakow, Kalwaria Zebrzydowska).

November 1st - All Saints' Day (Wszystkich Świętych)
There is no Halloween in Poland but the All Saints' Day is celebrated by almost everyone. People visit cemeteries, light candles and bring flowers to commemorate their beloved ones. It's a bank holiday. Restaurants are open and so are pubs but no crowds and parties should be expected. Museums are usually closed. 

November 11th - Independence Day (Święto Niepodległości)
National holiday, supermarkets are closed, restaurants and pubs are open as usual. In Poland this day is not really celebrated by masses, it's rather a matter of state - like leaving flowers at the Tomb of the Unknown Soldier in Warsaw. Ordinary people treat this holiday as just an additional day off. Museums are usually closed. 

November 29th - "Andrzejki" = the Eve of St. Andrew's Day

Although Poles don't celebrate Halloween they do like various kinds of spooky things ;) This is why the night before St. Andrew's Day (Andrew in Polish is "Andrzej") is believed to be a perfect time for fortune telling. Poles usually celebrate this night at home parties (sometimes dressed up as witches etc.) or go to clubs and pubs. The most popular way of fortune telling is pouring hot wax onto cold water through the key hole - the shape formed on water tells something about your future life. Celebrations usually get even more intense if the party actually includes at least one man named Andrzej (but this is not essential to keep everybody in a partying mood). "Andrzejki" is not a bank holiday.  

Christmas

December 24th - Christmas Eve (Wigilia)
It's not really a bank holiday so people go to work. But shops are usually open just to the early afternoon and many people take this day off (if they can). Just after dark there is a traditional meal at every home but first there is a custom of exchanging greetings and sharing special white wafer (something like altar bread). Then, after the meal, it's the time of exchanging presents which are traditionally placed under the Christmas tree. For many people the day of Christmas Eve is even more important than the Christmas Day itself. Many people attend the watch-night service at churches. 

December 25th-26th - Christmas Days (Boże Narodzenie)
It's a bank holiday and many pubs can be closed but restaurants are usually open but not in the morning. People go to churches and then spend time with families and friends, and eat lots of food.  Museums are usually closed. 

Other bank holidays in Poland: Pentecost (Sunday, 7 weeks after Easter), The Assumption of Mary (August 15th).