Despite being celebrated so close to Halloween, there is nothing ghoulish about the Day of the Dead (or el Día de Muertos as it’s known in Spanish), nor is it a sombre occasion. Families don’t get together to mourn their dead, but rather to celebrate their life. They make altars for their loved ones, or visit their graves and decorate the gravestones, often even having a picnic at the graveside.
It is said that the spirits of the dead come back to earth for one day, first babies and children who have died and later the adults. The festival coincides with All Saints’ Day (1st November) and All Souls’ Day (2nd November), but like many dates in the Christian calendar, the festival has its roots much further back in time than the arrival of Christianity in Mexico, dating back to the Aztecs.
It was a festival to honour the Goddess of Death, Mictecacihuatl, and originally lasted for the whole of the 9th month of the Aztec calendar (from around mid July to mid August). The Spanish conquistadores tried to eradicate the festival, but the Aztecs clung tightly to their beliefs.
Eventually the festival was reduced to just two days and was moved to coincide with appropriate dates in the Christian calendar. However the celebrations still have a nod towards the original Aztec celebrations, and Mictecacihuatl, in the guise of a well-dressed skeleton, still plays an important role.
I also found this post about Day of the Dead in Poland.