Jesus Christ is suggested to have been born sometime in the middle of the year, during or around the year 5-6 B.C.E., but it wasn't till about 30-33 C.E. that he is suggested to have died. His teachings lived on and became increasingly popular. With his teachings, however, the church came to the realization that these gods of nature and earth and the spirits on Samhain and Lemuria that they were worshipping (much like the Egyptian gods I posted about three days ago), were actually demons and false gods.
It became a moral issue between understanding the "true religion" and sticking to their pagan origins and traditions. Many were even persecuted and hunted down by Catholic militants. (Note, that this and other forms of "Christian" behavior is not Biblically backed, as the Catholic church was heavily influenced by Pagan cultures around this time). The problem was that much and most of the world at that time was involved in pagan worship (also polytheism, which is the worship of many gods), and many didn't want to convert into Christianity if they had to give up their traditions. Likewise, just as many Christian militants attacked Pagans, Pagans attacked many Christians.
During his reign from 284-305 C.E., Roman emperor Diocletian, persecuted many Christians for their faith. In 299, Diocletian and Galerius (another emperor), performed in a ceremony (not unlike the ancient Druids during Samhain) where they tried reading the burnt entrails of the sacrificed animals to predict the future. He could not read them, and he blamed the Christians for this error. This began his campaign against the Christians.
After Diocletian's reign, there came the emperor Constantine in the 4th century, who was another pagan Roman emperor. However, it is said that he had a vision on the battlefield of Christ, with the symbol of a "X" and a "P" in the background, and this is the earliest known usage of a cross in Christianity (it goes much farther back into pagan roots and symbols, but thats another day and another story). His mother was a Christian, and after seeing this vision while at war, he converted himself to Christianity. This paved the way for many pagans alike to transition into a Christian lifestyle, but no, it didn't happen overnight. In 325 C.E., emperor Constantine summoned the Council of Nicaea, and they made plans to send missionaries all over, saving them (into Christianity), and baptizing them. They soon realized that steering the public away from paganism would not be easy. Many still dressed up in animal skins, painted faces and worshiped their old gods.
In fact, it was so hard that almost 300 years later in the 7th century, Pope Gregory I allowed for an even simpler transition. The pagan worshippers would be allowed to worship just as they had been doing, but only if they were worshipping under the name of the Christ. As I mentioned yesterday, about the celebration of Lemuria which was held on May 13, it became Christianized into All Saints Day. It wasn't until the 8th century that Pope Gregory III decided to move the original dates of the celebrations of Lemuria up to the time of the Samhain festival.
Thus, November 1st became claimed "All Saints Day", to honor those Christians who didn't already have a Saint's day of their own. This day was also called "All Hallow's Day" (as Hallow is an equivalent word to a Saint). The traditions of Samhain continued to be practiced on October 31st, and many called it "All Hallow's Eve", because it was the evening before All Hallow's Day. All Hallow's Eve is only a short variation off from being eventually called Hallowe'en.
They say the Roman Empire never fell. It simply became the Roman Catholic Church.