Wiki..
In 6 CE Publius Sulpicius Quirinius, the newly-appointed Imperial Legate (governor) of the province of Roman Syria, was tasked to carry out a tax census of the new province of Judea, one of the three territories into which the kingdom of Herod the Great had been divided on his death in 4 BCE. The Gospel of Luke links the birth of Jesus to the census:
In those days a decree went out from Emperor Augustus that all the world should be registered. This was the first registration and was taken while Quirinius was governor of Syria. All went to their own towns to be registered. Joseph also went from the town of Nazareth in Galilee to Judea, to the city of David called Bethlehem, because he was descended from the house and family of David. He went to be registered with Mary, to whom he was engaged and who was expecting a child.
Luke 2:15
FIFY