Halakhah su Zaccaria 14:9

(starting at ועכשיו) We gather in shul and we say five chapters of psalms from Lechu Neranena (Ch. 95) until Hashem malach yirgezu amim (Ch. 99) because they are about the future redemptive days. Therefore (like Radak says), a person says to his friend “Let us sing to Hashem,” and then [the next chapter we sing] “Hashem reigns,” that is to say that we’ll fulfill that which is written (Zecharia 14:9) “On that day Hashem will be One and His Name will be One.” For Shabbat is a hint to this time, to “The Day that is Entirely Shabbat,” and then we’ll sing a new song (Ch. 96) to Hashem. For all songs are in the feminine, and the song of the future [redemption] will be in the masculine, like the Midrash explain. Therefore we say these chapters.

He says “כתר²¹³The Keter, כתר, was an ancient beginning of the Kedushah, קדושה, or sanctification prayer. It is no longer said and Kedushah now begins with the word na'ariẓkha, נעריצך. It is part of the Sabbath and festival Musaf Service (see footnote 166).
The Kedushah said during the Musaf Amidah is different than the Sanctification recited in the Shaḥarit Amidah on the Sabbath (see footnote 17). The Musaf Sanctification dwells more sublimely on the majestic conception of the angels in heaven glorifying the Eternal King and it introduces Israel proclaiming in response to the angelic choir, the holiness, glory, unity, and sovereignity of God's name. The Shema, the congregation's confession of faith, is part of the Musaf Kedushah (see footnote 17, section (3)).
The Shema entered the Musaf Kedushah as the result of a sixth century persecution. The Byzantine Empire forbid Jews to recite the Shema in public worship. On Sabbaths and festivals government spies would enter the synagogues to make sure the Shema was not said in its regular place during the Shaḥarit Service. As a means of getting the Shema into public prayer, the reader would insert it into the Musaf Kedushah, a place where the spies would not expect it and thus not be aware of it. The Shema has remained a part of this Sanctification ever since.
The Kedushah is part of the third blessing of the Amidah and in the Musaf Service for the Sabbath it is made up of the following parts along with connecting sentences: Isaiah 6:3, Ezekiel 3:12, the Shema, Deuteronomy 6:5, and Psalms 146:10. On festivals there is an additional verse from Zechariah 14:9 following the Shema.
Hertz, op. cit., pp. 528-31; 816-19.”, “the crown”, just as in the Additional Service.²¹⁴Musaf, מוסף; see footnote 166.