The name “Septuagint” derives from “The Translation of the Seventy”

The translation is known as the Septuagint, derived from the Latin word for “seventy”, referring to the koine Greek translation of the Torah.

Josephus, Antiquities 12.2

“He (Ptolemy II Philadelphus) procured the law to be interpreted...  

(Demetrius said) …O King, thou mayest write to the High Priest (Eleazar) of the Jews, to send six of the elders, out of every tribe; and those such as are most skillful of the laws: that by their means we may learn the clear and agreeing sense of these (five) books” The "Seventy" refers to the 72 Jewish scholars who were placed in separate rooms, and asked to translate the Torah. All of them translated it in the exact same way with 13 famous instances in which they all veered from translating the text in a perfectly literal way due to a concern of misinterpretation. The 72 scholars finished the translation in 72 days. The Jews view this as a day of mourning, as the rich multi-layered Hebrew text filled with hidden nuances, had been simplified into a superficial tongue.