Great question Yacov!
First century Christianity was controlled and run by Jews. Christians were severely persecuted by the Roman empire and by 3rd century Emperor Constantine became a Christian and declared Christianity as the official religion. Then it became a fashion for pagans to embrace Christianty without any true repentance and conviction. They brought a lot of pagan practices into Christianity. By giving official status to Christianity, he brought internal peace to the Empire. A brilliant military commander, he also had the genius to recognize that after declaring Christianity the "state" religion (Constantine forced all the pagans of his empire to be baptized into the Roman Church), there was need for true union between paganism and Christianity. The corrupt Roman Church was full of pagans now masquerading as Christians, all of which had to be pacified. What better way than to "Christianize" their pagan idolatries. Thus, the Babylonian mystery religions were introduced by Constantine beginning in 313 A.D. The Constantine-led Roman Church was more than willing to adapt and adopt pagan practices in order to make Christianity palatable to the heathen. Constantine used religion as a political tool, totally devoid of any true spirituality:
Pagan rituals and idols took on Christian names (e.g., Jesus Christ was presented as the Sun of Righteousness [Malachi 4:2] replacing the sun god, Sol Invictus ).
Pagan holidays were reclassified as Christian holidays (holy-days).
December 25th was the "Victory of the Sun-God" Festival in the pagan Babylonian world. In the ancient Roman Empire, the celebration can be traced back to the Roman festival Saturnalia, which honored Saturn, the harvest god, and Mithras, the god of light; both were celebrated during or shortly after the winter solstice (between the 17th and 23rd of December). To all ancient pagan civilizations, December 25th was the birthday of the gods -- the time of year when the days began to lengthen and man was blessed with a "regeneration of nature." Moreover, all of December 25th's Babylonian and Roman festivals were characterized by 5-7 day celebration periods of unrestrained or orgiastic revelry and licentiousness.
Yes Christmas tree, Santa Claus and Easter Bunny too were borrowed from Pagan cultures. They have no basis in our New Testament. However our Christian leaders generally agreed that even though they have no Biblical basis for celebrating Christmas and Easter these festivals would help Christians to proclaim the message of Christianity to the world.
Even Jesus was not born on Christmas day! However Easter, the day of resurrection is accurate because they calculate according to the Passover festivel at that time.
Christians should not celebrate Halloween and it is strongly discouraged in born-again, Evangelical and fundamentalist Christian Churches.
The following reasons are given why Christians should not celebrate Halloween:
(1) Christians should not celebrate Halloween because its customs and practices come from an ancient, pagan (Celtic) festival of the dead. Therefore to participate in Halloween is to serve the devil;
(2) Christians should not celebrate Halloween because pagans and satanists still observe it today as an un-holy day;
(3) Christians should not celebrate Halloween because it is wrong to participate in something that is used for evil or has evil connected to it.
Valentine Day:
Cupid has long played a role in the celebrations of love and lovers. He is known as a mischievous, winged child, whose arrows who would pierce the hearts of his victims causing them to fall deeply in love. In ancient Greece he was known as Eros the young son of Aphrodite, the goddess of love and beauty. To the Roman's he was Cupid, and his mother Venus. Therefore it is definitely pagan.
Today, the Catholic Church recognizes at least three different saints named Valentine or Valentinus, all of whom were martyred. One legend contends that Valentine was a priest who served during the third century in Rome. When Emperor Claudius II decided that single men made better soldiers than those with wives and families, he outlawed marriage for young, single men -- his crop of potential soldiers. Valentine, realizing the injustice of the decree, defied Claudius and continued to perform marriages for young lovers in secret. When Valentine's actions were discovered, Claudius ordered that he be put to death. Other stories suggest that Valentine may have been killed for attempting to help Christians escape harsh Roman prisons where they were often beaten and tortured.
According to one legend, Valentine actually sent the first 'valentine' greeting himself. While in prison, it is believed that Valentine fell in love with a young girl -- who may have been his jailor's daughter -- who visited him during his confinement. Before his death, it is alleged that he wrote her a letter, which he signed 'From your Valentine,' an expression that is still in use today. Although the truth behind the Valentine legends is murky, the stories certainly emphasize his appeal as a sympathetic, heroic, and, most importantly, romantic figure.
Again there is no Biblical basis for celebrating Valentines day. In third world countries Christians lovers do not celebrate Valentine. Here in America and in Europe it has become a cultural thing.