The Letter of Aristeas is available to read @ https://www.ccel.org/c/charles/otpseudepig/aristeas.htm. It's not too long and seems to be a narrative of the Septuagint's development.
Ptolemy II hired 72 Jewish scholars who in 72 days translated the entire Old Testament. There were many translations for the Greek, but this legend underlines the sole Greek authorization of the Septuagint.
(Hillsdale College - Ancient Christianity)