EFRAT, Israel - Of all the memorable historical occurrences in the Torah, none is more significant than the day of the revelation at Sinai, when Israel received the Torah from G-d.  Nevertheless, unlike the date of the exodus from Egypt (Passover, celebrated on the 15th day of Nissan) and the date of the clouds of glory (Sukkot, 15th of Tishrei), the date of the Revelation is never specifically recorded within the Bible itself.  Why not?
 
Moreover, although our Sages in the Talmud inform us that the Biblical festival of Shevuot (Weeks) is actually the commemoration of the day of the revelation - the "Festival of the Giving of the Torah", as we say in the prayers of that day - when we go to the trouble of checking this out precisely with a calendar and Midrash, something doesn't quite add up.
 
As we know, Passover commemorates the Exodus from Egypt, which took place on the 15th of Nisan.  To find out which day of the week it happened on, all we have to do is remember that the 10th of Nisan, the day on which the paschal lamb was taken in preparation for the redemption, fell on Shabbat - which is why the Sabbath before Passover is called Shabbat HaGadol (the Great Sabbath).  Therefore five days later, the 15th, had to have been a Thursday.
 
Now, given that the only guidelines the Torah provides for designating the festival of Shevuot is to count seven full weeks (49 days) from "the day following the festival"
[Lev. 23:15], if the first Passover in history fell on a Thursday (Wednesday night), the count of 49 took place on a Wednesday night; hence the 50th day - when the festival of Shevuot was celebrated - had to have been a Thursday night and Friday.  This would be fine except for the fact that the Sages all agree that the Revelation at Sinai took place on Shabbat and not on Friday!  (Indeed, the Amidah of Shabbat morning features the words, "Moses rejoiced in the gift of his portion" - a reference to the gift of Torah which he received on the Sabbath.)  Therefore how can the Festival of Shevuot, which comes exactly fifty days after the first day of Passover, be celebrating the giving of the Torah, which was in fact given on the fifty-first day of our count?
 
Let's consider several different approaches.  The Magen Avraham (Rabbi Abraham Gombiner, 1637-1683), in his commentary to the Orach Chaim section of the Shulchan Aruch [Siman 263], explains that this seeming discrepancy 50th and 51st days of our count (the 6th and 7th days of Sivan) serves as our source that 'yom tov sheni of galut' (the second day of the festival in the diaspora) actually has its roots in the Torah.  After all, throughout the Diaspora we have a second day of Shevuot - the seventh of Sivan, and the 51st day from Passover - which turns out to be when the Torah was actually given. When we remember that the Torah was indeed given in the desert and not in Israel, it makes sense that we received it on the second day of the Festival, celebrated throughout the Jewish diaspora.  Hence we have an ingenious source - a Biblical source, no less - for the institution of the second day of the festival in the diaspora (the Talmud in Beitza, 2b and 3a, explains the second day in terms of the Jews of Babylon not always knowing when the month began and when the Festival was supposed to be celebrated).
 
Fascinatingly enough, the Shelah HaKadosh (R. Isaiah Horowitz, 1565-1630) gave a reason for the second day of the Festival in the diaspora which fits in very nicely with the Shevuot reckoning.  He argues that life in the diaspora - because it is based upon gentile customs and a gentile calendar - is far more removed from Jewishness than is life in Israel.  Hence it is twice as difficult in the diaspora to feel the exodus, to experience Divine Protection, to sense the revelation, than it is in Israel.
 
From this perspective, the Book of Ruth which we read on Shevuot merely confirms the hardships of remaining Jewish outside of Israel, and thus silently confirms the need for a second day of the Festival outside of Israel.  After all, the story of Ruth is not only the tale of a sincere Jew-by-choice who becomes grandmother to King David, progenitor of  the Messiah.  The book opens when Elimelekh (a nobleman in Israel whose name means, "G-d is my King") leaves famine-ridden Bethlehem in search of greener pastures in Moab.  He soon discovers that his decision to leave Israel was a disaster. His two sons, Machlon and Kilyon (whose names mean "illness"
and "destruction") marry Moabite women and die before producing any heirs.  He may have saved some money, but he sacrificed Jewish continuity.  And so this not untypical family that leaves the `house of bread' ends up encountering a `world of death and illusion'. 
 
The tale of Elimelech can be seen as a description of what happens to a Jewish family when they attempt to embrace the Diaspora's values.  Ironically, if not for Ruth it would have been the end of Elimelech's line forever, the Jew who left Israel doomed to historic oblivion. Ruth's decision is the mirror-image of that of Elimelekh, her ill-fated father-in-law.  He left his homeland to embrace Moab, Ruth leaves Moab to embrace the people and the G-d of Israel.
 
And so to counter the threat of assimilation that always hangs over a family in the Diaspora, the Torah has provided an extra protective measure, the second day of yom tov.
 
A second reason why the exact date for the revelation is not revealed - and perhaps not even celebrated - is to save the Jews embarrassment for a failed experience. We know that only two days after the miraculous events of Sinai the Israelites soon succumbed to the temptations of the golden calf and returned to the heat of idolatry. 
Apparently G-d gave them His gift too soon - before they were really equipped to adequately appreciate it.  The Bible, therefore, does not eternalize the day of the Revelation.  Shevuot is merely an agricultural Festival - the celebration of the first fruits - and Biblically speaking it only coincidentally works out to fall on the day before the Revelation at Sinai.
 
Rabbi Samson Raphael Hirsch gives a third reason suggesting that the Torah specifically wants us to purposefully celebrate the Revelation a day before it actually occurred - in order to emphasize the cardinal importance of "the day before". 
Ordinarily when an important event is about to take place, only those behind the scenes know how much preparation has gone into the event.  For the guest, all that matters is what he experiences at the moment the invitation told him to appear. 
 
But for the families and all those involved in preparing a `great event', the months of careful planning are what truly counts and will determine the proceedings of the evening.  This is especially true with regarding to the receiving of the Torah: without adequate preparation, without going through the forty-nine steps of purification leading up to the final climax of the day before, the Torah that descends from Sinai won't find an adequate vessel to contain its infinite blessings.  Lack of adequate preparation caused a tragic foul-up the first time.  It is crucial that it never happen that way again.