All my life, growing up as a JW in the 1950s and 60s, I was taught the standard reasons Jehovah's Witnesses do not celebrate Christmas (while claiming to be Christian).
1. Jesus Christ was not born on December 25. The Witnesses claimed that Jesus was born in October. They didn't celebrate Christmas then either.
2. The date of Jesus' birth is not found in the Bible. They claim the only event in Jesus' life that occurred on an identifiable date was The Last Supper which was followed by his death.
3. Christmas is the celebration of a birthday. JWs do not celebrate birthdays (and claim the early Christians did not celebrate birthdays either).
4. They assert that the customs associated with the celebration of Christmas (including the date) have a Pagan origin.
There are, of course, very reasonable answers to all these objections. But none of the four reasons listed above is the REAL reason JWs must shun Christmas. The real reason is this:
Christmas is the ultimate testimony to and representation of the Incarnation -- God become man in the person of a Babe in Bethlehem. JWs reject the Incarnation.
Now JWs have a silly tale (unsupported by scripture, BTW) that the babe born in Bethlehem was the "transferred life" of the Archangel Michael, whom they claim was God's first creation. Surprisingly, at least to the Catholics I know, JWs at the same time claim to believe in the Virgin birth. They accept the part that the babe was conceived by Holy Spirit and born in Bethlehem without the aid of a human father, but they reject that this child is God become man.
JWs also claim that the adoration of the Infant is demeaning to Jesus Christ. They state that it detracts from his current rulership as the "King of God's Kingdom". Of course, everything about the JW explanation is demeaning to Jesus. And the angels who sang from heaven at the birth of Our Lord appear to disagree mightily with the JW opinion.
So the next time a JW wants to explain to you all the reasons they do not celebrate Christmas, ask that same JW how it is that they claim to be Christian while not acknowledging the miracle of the Incarnation.
And you might let them know that the Devil did not send the star, either.