The third day of slichos has a special quality not found in the other days of slichos, a quality that was hidden for generations. It was revealed by each Rebbe only to the one who would become the next Rebbe.
On the third day of slichos [gimmel slichos], the Alter Rebbe would say a Maamar just to his son Dov Ber who would eventually become the Mittler Rebbe. When the future Tzemach Tzedek became old enough, he was invited to join Rabbi Dov Ber in hearing the maamar from the Alter Rebbe.
The Tzemach Tzedek continued the tradition by saying a Maamar on the third day of Sichos to his son, the future Rebbe Maharash. And the Rebbe Maharash, in turn, said a Maamar to his son, the future Rebbe Rashab who would follow suit by saying a maamar on gimmel slichus to his son the Previous Rebbe.
Each Maamar was recited on the third day of slichos, and each one who heard it was cautioned, in the strictest terms, not to tell anyone that they had heard a Maamar that day. Therefore, the practice [and the specialty of the third day of slichos] remained a secret for almost 200 years until the time of the Previous Rebbe.
Our Rebbe points out that, at first glance, it is a wonder that the Previous Rebbe revealed this tradition to the public. After all, the select individuals who heard the maamar were not allowed to reveal it even to the people closest to them.
However, similar occurrences can be found throughout the Torah beginning with the generations leading up to the destruction of the Holy Temple. For example, the souls of the Tanaim were imbued with greater revealed soul powers than the generations who followed, and so too, the Amoriaim, the Geonim, and Rishonim in their turn.
Even though the earlier generations were on a higher spiritual level, each successive generation added new explanations and insights to the Torah. Given that earlier generations were spiritually higher, how is it possible for later generations to add to our Torah knowledge?
Our sages provide a metaphor of a dwarf standing on the shoulders of a giant who can see even farther than the giant. We are like the dwarf on the shoulders of a giant and as a result, our weaker generation can uncover new insights and explanations in Torah. Furthermore, we have an obligation to deepen and broaden our learning in all areas.
True, we don't have the merits and holiness of earlier times but still and all, we have the responsibility to clarify and refine our portion of the world by selecting the good sparks from the bad. For that reason, we are given a sword for victory, extra strength and concentration power to prevail over the darkness of the world to accomplish our mission.
Just as we are able to reveal new insights in the Torah in general, we have witnessed revelations in the inner secrets of the Torah. These concepts were previously hidden from everyone except for special individuals. Finally, the Arizal decreed that now it is a mitzvah to reveal this wisdom. We accomplish it by sharing the inner teachings of Torah through Chassidus.
Even though the third day of slichos was so precious that it was guarded as “top secret" and its unique quality was only revealed to the leaders of the generation, nevertheless each Rebbe would infuse the souls of his generation with the special power of the third day of slichos.
However, when B'nai Yisrael needed additional strength to accomplish their service, the Previous Rebbe revealed the greatness of Gimmel Slichos. That revelation was through the Yechidah of Torah-- Chassidus, the soul of Torah as it is unified with Hashem. And that Yechida of Torah, is bound to the Yechidah of the Jewish soul, making it easier for each Jew to accomplish his particular mission.
One of the special qualities of gimmel slichos is that it is always falls on Tuesday, the day which the Torah calls “good” in two different verses. This indicates that Tuesday is good for Heaven and good for creation. On all the days of slichos, we ask Hashem for forgiveness so we can have a good year. But on gimmel slichos, a double good is manifest, therefore, our davening accomplishes at least twice as much as on the other days of slichos.
When the Previous Rebbe saw that it wasn't enough just for the leader of the generation to be empowered to beneficially affect the Jewish people through his gimmel slichos, he found a more efficacious way to bring its good about. He publicized it so that all of us, men, women, children, old and young would use gimmel slichos to accomplish good benefits for the coming year. And our service on gimmel slichos accomplishes the that Hashem will give us and Klal Yisroel double good on Rosh Hashanah, Shabbos Tshuva, Yom Kippur and throughout the year.
Those of us poor in daas would add that our service of Teshuva Ila'a on Shabbos Tshuva magnifies and brings down blessings from all of our previous service.