So the Septuagint is claimed to have been translated between 285-246 BC during the reign of Ptolemy II Philadelphus of Alexandria, Egypt. His librarian, supposedly Demetrius of Phalerum, persuaded Philadelphus to get a copy of the Hebrew Scriptures. Then the Scriptures (at least Genesis to Deuteronomy) were translated into the Greek language for the Alexandrian Jews. This part of the story comes from early church historian Eusebius (260-339 AD). Scholars then claim that Jesus and His apostles used this Greek Bible instead of the preserved Hebrew text.
So lets look closer look at the 'Letter of Aristeas':
The whole argument that the Hebrew scriptures were translated into Greek before the time of Christ rests upon a single document. All other historical evidence supporting the argument either quotes or references this single letter.
In this so-called Letter of Aristeas, the writer presents himself as a close confidant of king Philadelphus. He claims that he persuaded Eleazar, the high priest, to send with him 72 scholars from Jerusalem to Alexandria, Egypt. There they would translate the Hebrew Scriptures into Greek, forming what we now call the Septuagint.
Jewish historian Josephus, Jewish mystic Philo (both first century AD) and others add to the story. Some say the 72 were shut in separate cells and "miraculously" wrote each of their versions word-for-word the same. They say that this proves "divine inspiration" of the entire Septuagint.
Thus, the Septuagint is claimed to exist at the time of Jesus and the apostles, and that they quoted from it instead of the preserved Hebrew text. But if as we shall see, it was not even written before Christ and the apotles, how could that be.
The verifiable facts:
The writer of this letter, Aristeas, claims to have been a Greek court official during the time of Philadelphus' reign. He claims to have been sent by Demetrius to request the best scholars of Israel to bring a copy of the Hebrew scriptures to Alexandria to start the Septuagint translation project. He even goes so far as to give names of Septuagint scholars, yet many of the names he gives are from the Maccabean era, some 75 years too late. Many of them are Greek names, definitely not the names of Hebrew scholars. There are many other evidences that this letter is from a different time period, and is thus a fake. The writer is lying about his identity.
The supposed "librarian," Demetrius of Phalerum (ca. 345-283) served in the court of Ptolemy Soter. Demetrius was never the librarian under Philadelphus.
The letter quotes the king telling Demetrius and the translators, when they arrived, how wonderful it was that they came on the anniversary of his "naval victory over Antigonus" (Aristeas 7:14). But the only such recorded Egyptian naval victory occurred many years after Demetrius death, so the letter is a obvious fraud or forgery, much like the forged Donation of Constantine (Latin, Donatio Constantini) which was a forged Roman imperial decree by which the emperor Constantine I supposedly transferred authority over Rome and the western part of the Roman Empire to the Roman Bishop or Pope.