Rabbi Hershel Schachter
In Parshas Pinchas we read about the institution of semicha. Moshe Rabbeinu was instructed by Hakadosh Boruch Hu to place his hand on his student, Yehoshua bin Nun, in the presence of Klal Yisroel and to ordain him. The Gemorah understands that the placing of the hand was merely a horoas sho'ah which only applied to Moshe Rabbeinu and does not apply in later generations. The institution of semicha, however, is a din l'doros. One cannot be a member of the Sanhedrin unless he has proper semicha. The S'ridei Aish points out that just as Moshe Rabbeinu's conferring semicha on Yehoshua bin Nun had to be done in a public fashion, so too in the future generations semicha is always supposed to be conferred in a public fashion. The Ramah (in Shulchan Aruch) quotes from the Abarbanel that the semicha that we have today is only an imitation semicha since the din requires that semicha should be ish m'pi ish ad Moshe Rabbeinu and has to be conferred in Eretz Yisroel, and this chain of masorah was broken about 1500 years ago.
In the mid 1500's the rabbonim who lived in Tzfat gathered together in order to confer semicha on Rav Yosef Caro and several other rabbonim. This was based on the suggestion made by the Rambam that perhaps there is an alternate route for conferring semicha: either one who is himself has semicha can confer the semicha on another talmid chacham and that will be ish m'pi ish ad Moshe Rabbeinu; or if all of the rabbonim in Eretz Yisroel gather together perhaps that is another way of conferring semicha. The mikubolim of Tzfat in the sixteenth century were motivated to attempt to reinstitute the semicha because they were hoping to live to see the coming of moshiach. The Gemorah interprets the possuk at the end of Malachi to mean that Eliyahu Hanovi will come and appear before the Sanhedrin, who will reconvene in the location that they last met before they were disbanded, to announce the coming of moshiach the next day. This is the reason why the mikubolim chose to live in Tzfat at that time as opposed to Jerusalem because the last location where the Sanhedrin met was clearly in the Galil - Tiveria, Tzipori, Usha, S'farom, etc.
If no one today has bona fide semicha, as the Ramah quoted from the Abarbanel, then you cannot have any Sanhedrin. How, then, can the prophecy of Malachi be fulfilled? Eliyahu Hanovi will not be able to appear in front of the Sanhedrin! For this reason, the rabbonim in Tzfat reinstituted semicha in the hope that someday there would be seventy-one properly ordained rabbonim who would be able to constitute a Sanhedrin and be able to facilitate the coming of Eliyahu Hanovi to announce the coming of Moshiach.
The end of the story was that fewer than ten rabbonim received semicha and the entire plan fizzled out. Even though today the semicha is only imitation semicha, nonetheless, Rabbi Meir Shapiro of the Lubliner Yeshiva instituted that the imitation semicha be conferred in a public fashion. He originated the concept of a Chag Ha'semicha which our Yeshiva still follows.
Semicha in the days of the ta'noim had to be conferred by a beis din of three who did so as representatives all of Klal Yisroel. The Rambam writes in his commentary on Mishnayos Bechoros that it is for this reason that semicha had to be conferred only in Eretz Yisroel because the concept of Klal Yisroel only applies to those Jews who live in Eretz Yisroel. This is probably the reason why semicha must be conferred in a public fashion because the halacha considers it as if all of Klal Yisroel is conferring the semicha.