We, Spanish, believe that any excuse is good for a party, so we are happy to adopt other cultures' traditions if they involve some fun. This is why we go along and are buying fancy dresses and loads of sweets. At present the "trick or treat" exchange has not yet fully become a tradition here, but it might happen in the future.
But in Spain what we really celebrate is the 1st of November, a national holiday called "Dia de todos los Santos", all saints day. We use this day to remember our loved ones by going to the cementery to pray and to bring flowers to our relatives and friends that have passed away. Flower shops' make a year earnings in one day.
We do not do it as the Mexicans' which will nearly have a party at the cementery, and we do not paint our faces, or we believe the world of the death is closer to the world of the living on this particular day. We just simply remember them and the majority of people will also attend mass, all a lot more serious than in other cultures.
It is quite clear that all those believes, however we would like to celebrate them, have a common ground: celebrate death once a year, instead of life. Bearing in mind a lot different cultures do the same on the same day (or night) makes me think there was some kind of pagan tradition than the different religions adapted for themselves, and nowadays people is adapting to our times, in a more fun way.
Regardless of how you choose to celebrate the next bank holiday, I hope you have a good one!