The origins of what is referred to as a 'day of Christian solemnity' are somewhat mixed, though it's more often agreed upon that these traditions originated in the far east of the Roman Empire in the 4th century CE. At the time, however, these commemorations of Christian martyrs usually took place around April and May, and the Eastern Orthodox Christians similarly observes All Saints' Sunday after Pentecost in late May/early June. Although this custom had become fairly mainstream in Central and Western Europe by the 5th and 6th century CE, it did not become official until Pope Gregory IV made All Saints' Day an authorised holiday in the Catholic Church in 835 CE.
The reasons behind November 1st becoming the designated date are heavily influenced by the customs of Anglo-Saxons, Franks and Celts, specifically those that coincided with the end of the harvest season and beginning of the 'darker-half' of the year. This period would later evolve became known in western European traditions as 'Allhallowtide', which began on October 31st with 'All Hallows Eve' (now 'Halloween'), followed by All Saints' Day on November 1st, and concluded on November 2nd with All Souls' Day - a separate day of observance devoted to all other deceased individuals. It should be noted that the Catholic Church's success as a major world religion is often attributed to its ability to adapt local traditions to Christian teachings, and these pagan customs are no different. We also suppose they didn't want any holidays interfering with Easter celebrations!
In newly-christened Poland, both All Saints' and All Souls' Day was referred to as Dziady (ENG: Forefathers), a pagan Slavic belief that the souls of ancestors would come back to earth to visit their families. Small bread loaves called powałki (later 'heretyczki') would be prepared several days in advance as offerings to these returning spirits. Fire was believed to have a particular draw on dead souls, which is why bonfires would be set at major crossroads in between community areas. These would guide wandering souls back to their earthly homes, as well as offering warmth on their journey. The living would place the aforementioned bread loves at their fireside as a returning treat! Family members would also visit cemeteries and put food on graves as additional offerings, which would later evolve into lit candles as Christian influences began to seep in. It's this particular aspect of the tradition that has remained into the present day!
All Saints' Day on November 1st is one of Poland’s most important public holidays, so much so that it was even allowed during the country's communist period (remember, communist ideology typically believes in 'state atheism'). Only transport and emergency service staff are expected to work – don’t be surprised to find your favourite hostelry bolted shut for the night. Since All Souls’ Day on November 2nd is not a bank holiday, Poles will use November 1st to pay respects to both deceased family and friends and saints as well. For this reason, you'll see whole families descend on graveyards to lay wreaths and light candles for deceased family members, and prayers said at the gravestone are meant to help the souls of the dead. It's fair to say that devoutly-catholic Poland takes these duties particularly seriously and even the graves of the unknown or forgotten are cleaned up and bedecked with candles. As an outsider, you may not think lurking around a cemetery in the dark is the best way to spend an evening, but it’s incredibly beautiful to see the cemeteries lit up by candles all night long, so wrap up warm and go have a look. While we could wax poetic about the unearthly glow of the immense candlelight, the murmur of prayer and psalms, the subtle smells of the incense, fresh flowers and burning wax, the shades of ravens in the trees, the wet grass and mists, and the surreal duality of the supernaturally-charged, yet tranquil atmosphere, we’d prefer you just experience it for yourself. Remember to take a candle along with you (or buy a few outside the cemetery)!
The old pagan traditions that gave birth to Allhallowstide and Dziady, meanwhile, have evolved elsewhere. We've all heard of or witnessed Halloween, which takes place on October 31st and is very much associated with American culture today. It was actually introduced to the 'new world' by western Europeans and had become a major calendar event by the mid-19th century. Thanks to globalisation and American pop culture being as sticky as their confectionery industry, the American format of Halloween has come full circle and is becoming more and more common across Europe. That being said, Poland remains fiercely protective of its own traditions, which means the sea of candles at cemeteries around the country are here to stay for the long term!
For more specifics, read about All Saints Day in Warsaw, Kraków and Gdańsk.