From here (an essay based on the Likutei Sichot volume 24, pg 1-11).

The Talmud (tractate Sefer Torah, 1:8) says:

Seventy sages translated the Torah into Greek for King Ptolemy. That day was as difficult for the people of Israel as the day on which the [Golden] Calf was made; for the Torah could not be fully translated.

Read that essay for all the details, but in short:

• The Lubavitcher Rebbe points out that it doesn't say the day the Golden Calf was worshiped, it says the day on which the [Golden] Calf was made.

• The Golden Calf was made the day before it was worshiped, as Aharon told the Jews (Shemot 32:5) after he made the Golden Calf, "Tomorrow is a festival unto G-d!"

• Aharon did not intend to use the Golden Calf as an idol, the people woke up early the next morning and worshiped it (Exodus 32:6).

• So the day the Calf was made was a day of potential, it could have been good, it could have been bad (at the end of the day it turned out to be bad)

• So too, the day the Torah was translated into Greek by Ptolemy's decree had the potential to be good or bad, and in the end, the negative possibilities of the endeavor were averted, or at least greatly minimized.

• the deeper meaning of the Talmud's words that the "difficulty" lay in that "the Torah could not be fully translated." Had the seventy sages fully -- that is, precisely and exactly translated the Torah into Greek, it would have been exposed to misinterpretation and distortion. It was only because they succeeded in presenting Ptolemy with a less than literal translation that this tragedy was averted. Indeed, their translation yielded the positive result of bringing G-d's word to the Greek world, and showing the way for the subsequent translators of Torah who would spread the light of Torah to all peoples and cultures of the earth.

The essay also analyzes another statement in the Talmud, (tractate Shabbat Shabbat 13b and 17a):

A count was conducted, and it was found that the sages of Shammai were more numerous than the sages of Hillel. Eighteen ordinances were enacted on that day... and that day was as difficult for the people of Israel as the day on which the [Golden] Calf was made.

From here (an essay based on the Likutei Sichot volume 24, pg 1-11).

The Talmud (tractate Sefer Torah, 1:8) says:

Seventy sages translated the Torah into Greek for King Ptolemy. That day was as difficult for the people of Israel as the day on which the [Golden] Calf was made; for the Torah could not be fully translated.

Read that essay for all the details, but in short:

• The Lubavitcher Rebbe points out that it doesn't say the day the Golden Calf was worshiped, it says the day on which the [Golden] Calf was made.

• The Golden Calf was made the day before it was worshiped, as Aharon told the Jews (Shemot 32:5) after he made the Golden Calf, "Tomorrow is a festival unto G-d!"

• Aharon did not intend to use the Golden Calf as an idol, the people woke up early the next morning and worshiped it (Exodus 32:6).

• So the day the Calf was made was a day of potential, it could have been good, it could have been bad (the next day it turned out to be bad)

• So too, the day the Torah was translated into Greek by Ptolemy's decree had the potential to be good or bad, and in the end, the negative possibilities of the endeavor were averted, or at least greatly minimized.

• the deeper meaning of the Talmud's words that the "difficulty" lay in that "the Torah could not be fully translated." Had the seventy sages fully -- that is, precisely and exactly translated the Torah into Greek, it would have been exposed to misinterpretation and distortion. It was only because they succeeded in presenting Ptolemy with a less than literal translation that this tragedy was averted. Indeed, their translation yielded the positive result of bringing G-d's word to the Greek world, and showing the way for the subsequent translators of Torah who would spread the light of Torah to all peoples and cultures of the earth.

The essay also analyzes another statement in the Talmud, (tractate Shabbat Shabbat 13b and 17a):

A count was conducted, and it was found that the sages of Shammai were more numerous than the sages of Hillel. Eighteen ordinances were enacted on that day... and that day was as difficult for the people of Israel as the day on which the [Golden] Calf was made.