This tradition is very old, and has been happening in Poland for more than 1,000 years! It also is a time to bring family members together and remember loved ones who are no longer alive. All Saints' Day is also celebrated in other countries, too!
The type of flowers placed on the graves depends on the part of the country the family is from. Different flowers are used in the north and the south of Poland; or, if you live on the seaside or in the mountains. However, the holiday is celebrated similarly all across the country.