The Haftora for the second day of Shavuoth is from our prophet Habakkuk. He says a prayer, and tells of many great miracles that Hashem did for the Jewish people, starting with the giving of the Torah. He is pained by the suffering of the prolonged exile, and finally has a vision of the final redemption, and he is filled with joy.
The simple reason for reading this Haftora is because it mentions the giving of the Torah, but there has to be more.
This Haftora is only read outside of Israel, where we have a second day of Shavuoth. The reason for this extra day is not the same as the extra days added to Pesach and Sukkos, they were added because of a doubt when the holiday was. Because in the times that we would sanctify the new month according to the testimony of two witnesses that saw the new moon, the community outside of Israel wouldn't know until a while later. So the rabbis Instituted an extra day, just in case the new month began the next day. Shavuoth, on the other hand, is not because of a doubt, it always falls on the fiftieth day of the Omer. Rather, the extra day is because the rabbis wanted that all the holidays have the same laws. So while the other holidays have an extra day, because maybe, that is the actual holiday, on Shavuoth, the Jewish people add an additional day of their own will, and take a day that is undoubtedly mundane and make it holy.
This is in fact the essence of the day and it captures the essence of Shavuoth as well.
Shavuoth is the day we received the Torah, it is the day our mission began. It is the day that Hashem gave us the ability to do our mission. The Midrash says, that before the giving of the Torah, above and below, spiritual and physical, didn't mix. At the giving of the Torah all that changed, as it says, "And Hashem descended on Mount Sinai." Now there is the ability to mesh G-dliness with the physical, to make this world into a dwelling place for Hashem. We do this by learning Torah, doing mitzvahs and using the most mundane parts of our lives to serve Hashem. Our mission began at the giving of the Torah, and it ends with the coming of Moshiach.
Habakkuk was given a vision of future of the Jewish people. He saw all of our suffering in exile, and he couldn't take it. He pleaded with Hashem, and he outright demanded on behalf of the Jewish people. But when he saw the time of Moshiach, he understood and was happy. That is why he said this prayer, to ask forgiveness for the harsh words he spoke.
In his prayer, he begins telling of the giving of the Torah, then he goes through many of the good things Hashem did for the Jewish people, then he tells of the exile, he alludes to the difficult war of Gog and Magog, that will happen right before the coming of Moshiach, and finally, he rejoices with his coming.
In other words, he is telling us that what Hashem does is good, and even if we don't see it that way, because of the harsh exile, he knows the truth, because he saw the future redemption, and it is all worth it. Just as he rejoiced, so will we.
On the second day of Shavuoth, the day that we turn from mundane to holy, we received the Torah, with which we make the world into a dwelling for Hashem, mundane to holy, and we read the Haftora about the completion of our mission, the coming of Moshiach, when the world will be a home for Hashem, the whole world will go from mundane to holy. And that is the connection between the second day of Shavuoth and the Haftora.
At the beginning of his prayer, Habakkuk says, "I heard your message and I was afraid." This verse is quoted by Rabbi Shimon bar Yochai in the Zohar, and this passage of the Zohar is part of the Tikkun Leil Shavuoth, that we read as we stay awake on the first night of Shavuoth. "Rabbi Shimon rejoiced and said, '"I heard your message and I was afraid," at that time it was appropriate to have fear, but in our case it all depends on love.'" Being that we read this in the synagogue on the second day of Shavuoth, means that it applies to us. Why does Rabbi Shimon say, that "In our case it all depends on love?"
Habakkuk said that he heard and he was afraid. Hearing is not like seeing, hearing implies distance and less understanding than seeing, therefore it is associated with fear. Seeing, on the other hand, implies closeness and deep understanding, hence it is associated with love.
We see this differentiation when it comes to studying Torah. The written Torah is associated with fear, as it says in Maseches Sofrim, "A face of fear for reading (Torah)." Why fear? Because it is distant, there is a lack of understanding, as we see, that even if one is just reading the words of the written Torah, even if he doesn't understand what he is saying, he is obligated to recite the blessing for learning Torah. On the other hand, learning Talmud, the oral Torah, is associated with love, because it must be understood.
Habakkuk heard and felt distant, therefore, he was filled with fear. Rabbi Shimon bar Yochai was close to Hashem, and his experience was of seeing, as he said, "I see now what no man has seen since the day that Moshe went up a second time on Mount Sinai."
Each of us stood at Mount Sinai, saw the great event and the greatest levels of G-dliness. Therefore we are close and our experience is one of love as well.
He continues his prayer and he talks about the power of the Aron, the Ark of the Covenant, which housed the tablets of the Ten Commandments and according to some, the Torah that Moshe wrote. The Ark symbolized the Torah. He concludes that part, "The ways of the world are His."
The Talmud concludes with a teaching on these words of Habakkuk, and it is recited in our morning prayer. "It was taught in the school of Eliyahu, 'Whoever learns (Torah) laws every day, it is certain that he is (going to receive a portion in) the world to come, as it says, "The ways (halichos) of the world are his," don't say halichos (ways), but halachos (laws).'" Why specifically laws? Why not the study of Torah in general?
The general study of Torah has many opinions. There are 70 ways of understanding the Torah, or 600,000 ways. But there is a point where there is no division, and total unity of Hashem's will in Torah. This place has only one opinion, Hashem's essential will. This is not expressed by the general study of Torah, which has many opinions. This essential oneness of Hashem, beyond any division, is expressed in halacha, where we have one bottom line that we follow. Since it is the study of laws that brings the essential will of Hashem into the world and into practice, it guarantees one a place in the world to come.
Even more, through studying halachos, "the ways of the world" become "his." He becomes a master over worldly matters.
Habakkuk then speaks of the exile, "For the fig tree will not blossom, and there won't be food growth on the vines..."
Then he became happy with the realization that Hashem will help us. "Yet, I will rejoice in Hashem, I will be joyful in the G-d of my salvation."
What is he so joyous about? He continues, "G-d Hashem will be my army, He will make my feet as a deers, and He will lead me on high places, to the Choirmaster with my songs."
"G-d Hashem will be my army," meaning, Hashem gives us the strength to be victorious over the exile and bring Moshiach.
"He will make my feet as a deers." Why does he refer to feet? Because he is talking about the end of the exile, which is called, "ikvisa d'mishicha," the footsteps of Moshiach, when the way we serve Hashem is not so much like the head, with deep understanding, rather similar to the feet, that will go and do things that the head will never do. The feet symbolize serving Hashem through self sacrifice, which is the calling of the last generation before the coming of Moshiach, our generation. They will be like a deer's feet, which can go far with ease. Meaning, that Hashem will give us the strength to win this war and end the exile, with ease.
"And He will lead me on high places," meaning, that we will be lifted to the highest levels, when Moshiach comes.
"To the Choirmaster (lamnatzeach)." Lamnatzeach could also mean, "to the victor," which in this case will be Hashem and the Jewish people. "With my songs," is our song, as we will break out in song, as it says about the coming of Moshiach, "And there we will sing a new song."
May we merit to rejoice, just as Habakkuk did, with the coming of Moshiach. May it happen soon.