Halloween traditions can be traced back to the Celtic festival of Samhain. Samhain was a pagan religious festival that is usually celebrated from October 31 until November 1. This was when the Celtics would welcome the harvest and what they referred to as "the dark half of the year".
The Celtics that celebrated this holiday believed that this is when the barriers between the physical world and the spirit world would break down. They believed that this would allow more interaction between humans and those who inhabited what they considered to be the Otherworld.
Since the Celtics believed that the barrier would be broken down during this time, they would light a large bonfire in what would now be considered the town square. They would throw crops and animals into the fire as sacrifices to the deities to help ward off monsters. They would also dress up in costumes, most likely animal skins and heads, and try to predict the futures and fortunes of others.
Once the Roman Empire conquered and acquired the majority of Celtic territory, two Roman festivals were combined with the Samhain festival. The first festival was called Feralia. This was typically in late October when the Romans would commemorate the passing of their dead. The second festival was a day to honor the Roman goddess of fruit and trees, Pomona. This is most likely where the tradition of bobbing for apples came from since the symbol of Pomona is an apple.
In the colonies of soon-to-be America, the celebration of Halloween was extremely limited because of rigid Protestant belief systems. Halloween was more common in the southern colonies. As time passed and beliefs, along with customs, began to mesh, a distinct version of Halloween began to come about in America.
The first celebrations were public events that were held in order to celebrate the harvest and the incoming of crops. These were called "play parties," and all those who were there to celebrate would share stories of the dead, attempt to tell fortunes, and would dance and sing.
Halloween, however, didn't become very popular until the second half of the 19th century when there was a large influx of immigrants. Among these immigrants were millions of Irish. The Irish that were fleeing to America from the Irish Potato Famine are largely what helped to popularize and make Halloween more of a national holiday.