Halakhah sa Habakuk 3:19: Tur & Shulchan Arukh, Orach Chayim

The holiday of Shavuot is on the fiftieth day of the counting of the Omer. The order of prayer is like that on the holiday of Passover, except that we say, "this festival day of Shavuot, the time of the giving of our Torah." And in the additional service (musaf), we mention the additional sacrifices [of this holiday], "And on the Day of the First Fruits," until, "and two daily sacrifices according to their law." And we recite the complete Hallel and take out two [Torah] scrolls: On the first day, five people read in Parshat Yitro [beginning] from, "In the third month" (Exodus 19:1), until the end of the [parsha]. And the one who concludes reads [beginning from], "And on the Day of the First Fruits" (Numbers 28:26); and he concludes with the chariot of Ezekiel, ending [that reading] with the verse, "Then a spirit lifted me up" (Ezekiel 3:12). And on the second day, we read the section [that begins with], "Every firstborn" (Deuteronomy 15:19), to the end of the section; and the one who concludes reads the same as [that which was read] the previous day, but he concludes with Habakkuk, [beginning] from, "But the Lord in His holy Abode" (Habakkuk 2:20), until, "for the leader; with instrumental music" (Habakkuk 3:19). And we have the practice in all places to say the Azharot, which is based on the sum total of the [Torah's] commandments, during the additional service after the repetition of the [standing amidah] prayer. But every place does what is in accordance with its custom.

On the second day, we read in the section (Deuteronomy 15:19) "Every firstborn" until the end of the portion. The Maftir is read like the previous day, and as Haftorah we read in Habakkuk from "And Hashem in His Holy Sanctuary" until "for the conductor with my melodies".