Wikipedia
Akdamut, or Akdamus or Akdamut Milin, or Akdomus Milin ( Aramaic: אֵקְדָּמוּת מִלִּין, "In Introduction to the Words," i.e. to the Aseret ha-dibrot, the Ten Commandments), is a prominent liturgical poem (piyyut) recited annually on the Jewish holiday of Shavuos by Ashkenazi Jews written in Aramaic. It was penned by Rabbi Meir bar Yitzchak ("Nehorai") of Orléans, who was a cantor (prayer leader) in Worms, Germany, (died ca. 1095). Akdamut consists of praise for God, His Torah, and His people.
Akdamut is read in almost all Ashkenazi synagogues on the first day of Shavuos during the Torah reading. The original practice was for it to be recited after the reading of the first verse (Exodus 19:1), but in the past few centuries, the practice has developed in many congregations (mainly Eastern European ones) that the poem is read after the kohen has been called to the Torah reading, but before he recites the blessing.
The reason for the original practice was that, from Biblical times to well into medieval times, each verse of the Torah reading in Hebrew would be followed by its interpretation into Aramaic, and therefore it would be appropriate, after the first Hebrew verse was read, for another reader to provide an Aramaic gloss including this "introduction". However, when the simultaneous Aramaic interpretation fell into disuse, the recitation of Akdamut remained between the first and second Hebrew verses, where it no longer seemed an appropriate interruption, so it was relocated to before the commencement of the Torah reading.
In most synagogues it is read responsively: the ba'al korei (Torah reader) singing two verses, and the congregation responding with the next two verses. Although it is considered "Judaism's best-known and most beloved piyyut", there are some synagogues where it is not recited.
Its adoption into the regular liturgy took some time; it is not mentioned as part of the Shavuos liturgy until the first decade of the 15th century and the earliest prayerbook to contain it was published in 1557. Apparently it replaced an earlier piyyut, Arkin Moshe, which was a folkloric poem describing the excitement among the angels when God brought Moses up to Heaven to receive the Ten Commandments. The adoption of Akdamut into the liturgy may have been assisted by a folktale that connected its composition with a miraculous event involving the defeat of an evil sorcerer monk who was using magic to kill countless Jews.