You’ll be glad to know that you have not missed it. Today is the eighth celebration of this great day, which was instituted in 2006 by the International Organization for Septuagint and Cognate Studies (IOSCS).
The rationale behind the date apparently comes from Robert Kraft’s observation that the date is the only one we have record of being historically related to the Greek Scriptures. In a document dating to February 8th, 533 C.E. the Emperor Justinian, announces permission for public reading of Jewish Scriptures in the Roman Empire. He proclaims his approval of any language, but where Greek is used he states that “those who use Greek shall use the text of the seventy interpreters [i.e. the LXX], which is the most accurate translation, and the one most highly approved…” An English translation of the novella is available here.
There are many reasons to love the Septuagint – not least of all the wide variety of options for pronouncing the word itself (sep-TOO-a-jint, sep-TOO-a-gint; SEP-too-jint; SEP-too-gint; SEP-twa-gint; and on and on). It is a greatly under-valued aspect of the heritage of the Christian religion, since it was (and is) an integral part of the formation of the Old Testament as we read it in our ESVs and NIVs. If we really want to access the most original form of Old Testament scriptures, the LXX is critical to that task since it often preserves older readings, as more biblical scholars are realizing. It also paved the way for the New Testament by furnishing an accessible corpus of scripture (and theology?) to draw upon for the apostles and early church. The LXX is an amazing artifact in its own right, too, a monument to the language, religion, and society of Ptolemaic Egypt. Oddly, more than being ignored in much of biblical scholarship, it is ignored by scholarship concerned with Hellenistic Judaism as well.
So, to help turn the tide, and in the spirit of the “most highly approved” Greek translation, consider reading an excerpt from the LXX today. PDFs of the New English Translation of the Septuagint (NETS) are freely available here. Or, you might consider reading an accessible introductory work such as T. Michael Law’s When God Spoke Greek (reviewed in many places, but currently receiving a “dialogic” review here), or Müller’s The First Bible of the Church: A Plea for the Septuagint.