Actually the legend is that the TORAH portion of the Septuagint was translated by approximately 70 scholars (actually 72, as it was 6 from each tribe). When each group had finished translating, they found that all the translations were identical. This is obviously a legend and not history, although most legends have a foundation of some history. Obviously someone did the translation.
The Torah section is a good quality translation. But the rest of it is not.
We have portions of the Tanakh written in Hebrew that predate the Septuagint. Perhaps the oldest archeological find is the Silver Scrolls which date to the 7th century BCE.
The fact that the Great Isaiah Scroll (one of the Dead Sea Scrolls) is almost exactly the same as the Masoretic texts proves how meticulously Jewish scribes copied the texts.
So yes, the Septuagint is a translation, and a bad one. I am not one to say that the Tanakh is somehow word for word revealed by God. I know perfectly well that its many authors were often edited, and spliced together, to form the Tanakh we have today. But if you are going to go for an accurate Tanakh, you have to go for the Hebrew. Not the Greek.
Yes, there is a day in the Jewish calendar that is traditionally observed as a day of mourning in opposition to the translation of the Hebrew Scriptures into Greek. This day is the 8th of Tevet.
The Talmud, in Tractate Megillah 9a, refers to the translation of the Torah into Greek as a calamitous event, comparing it to the sin of the Golden Calf, because it represented a significant change and potential dilution of the sacred text. The concern was that translating the Hebrew Scriptures into Greek could lead to misinterpretation and misuse by non-Jews, as well as the potential loss of the original nuances and meanings inherent in the Hebrew language.
Today we don't really share that horror of translations. True, we only consider the Hebrew to be the actual Tanakh. But there are many wonderful editions of the Tanakh that have the Hebrew on one page, and an English translation on the other.