Halloween is a celebration on the night of October 31st. It is most practiced in the United States and Canada. Children wear costumes and go to people's homes saying "Trick or treat!" to ask for candy (sweets in the UK) and people give it to them. The suggestion is: "Give me a treat or I will play a trick on you."  People traditionally dress up as ghosts, witches, or other scary things for Halloween. Halloween for adults is not the same as it is for children.  It is often referred to as ‘the scariest time of the year’.

For Christians it is the eve of All Saints' Day, which begins the three-day observance of Allhallowtide.  This covers the three days – October 31 (All-Hallows Eve or Hallowe'en), November 1 (All Saints) and November 2 (All Souls).  All Hallows' is a Christianized holiday and originated in Ireland. It also has pagan roots.

Origins 
Halloween originated from Ireland. The pagan holiday Samhain, which the All Saints holy day replaced, was also known as the Day of the Dead. Many wiccans and modern pagans celebrate the Day of the Dead. It is the day that some believed the souls of dead people come back to Earth. This is a happy holiday (even though it celebrates death) because some of the souls will visit the homes of their family. Pope Gregory III originally designated Halloween on November 1 as a time to honor all saints. Soon, All Saints Day incorporated some of the traditions of Samhain.

Many Lutheran churches celebrate a holiday on November 2 called the Reformation. This holiday celebrates the day that Martin Luther put The Ninety-Five Theses on a church door.

After mass Irish and Scottish immigration in the 19th century, Halloween became a major holiday in America.

The mystical rituals of earlier times evolved into more lighthearted fun and games. For example, the somewhat heavy concept of connecting to the dead was replaced with the more lighthearted idea of telling the future. Bobbing for apples, for example, became popular as a fortune-telling game on All Hallows' Eve: Apples would be selected to represent all of a woman's suitors, and the guy—er, apple—she ended up biting into would supposedly represent her future husband. In fact, Halloween previously posed a huge matchmaking opportunity for young women in the 19th century.

Symbols 

Development of symbols connected with Halloween formed with time. Jack-o'-lanterns are traditionally carried on All Hallows' Eve in order to scare evil spirits. Elements of the fall season, such as pumpkins, harvest, and scarecrows, are also common. Homes are often decorated with these types of symbols around Halloween. Halloween imagery includes themes of horror, magic and the supernatural. Black, and orange are Halloween's traditional colors.

Costumes

During Halloween some people, especially children, wear a costume. People have worn costumes at Halloween for centuries. Wearing a costume may come from Celtic festivals of Samhain and Calan Gaeaf. It could also be from the Christian Allhallowtide.

Early costumes were usually scary. They were often supernatural beings or from folklore. In the 1930s costumes of characters from literature, radio, or movies became popular, and such costumes are also popular today. Scary costumes are still popular.

Games and Other Activities 
In addition to trick-or-treating, there are several traditional activities connected with Halloween. 

 In old times people would try to tell the future, especially to try to learn who they would marry.
 People make Jack-o-lanterns and place them in front of their houses.
 People play a game called apple bobbing. In this game apples are placed in water, and people must try to remove the apples with only their teeth.
 Telling ghost stories, listening to Halloween-themed songs and watching horror films are common activities on Halloween. TV shows (with special shows usually for children) are commonly shown on or before Halloween, and new scary films are often released before Halloween.
 Visiting a 'haunted attraction'. These are places like houses, farms, or forests, which are decorated in a scary way, and where actors in costumes make scenes to scare the visitors.
 Decorating one's house and front yard. People will often decorate the front part of their houses with Halloween-themed symbols like ghosts, graves, and black and orange objects.

References

Other websites
Halloween's Bloody Legacy

Holidays