Most of the Mahzorim used among Sepharadim instruct reciting the "Yag Middot" – the list of God’s thirteen "attributes of mercy" ("Hashem Hashem Kel Rahum Ve’hanun…") – at the time when the Torah scroll is removed from the ark on Yom Tob. In truth, the propriety of this practice is subject to considerable controversy. The proponents of reciting the "Yag Middot" on Yom Tob base this practice on a comment of the Arizal (Rav Yishak Luria of Safed, 1534-1572) that one should recite the thirteen attributes on Yom Tob when the Sefer Torah is removed from the ark. This custom is approvingly recorded by the Hid"a (Rav Haim Yosef David Azulai, 1724-1806), in his Birkeh Yosef (Hilchot Pesah), and is also documented by Rav Yaakob Haim Sofer (Baghdad-Israel, 1870-1939), in his Kaf Ha’haim (133:11).
Furthermore, Rav Azriel Mansour wrote a lengthy essay on the subject in his introduction to the work Yimsa Haim, supporting this practice. He notes that this was the custom followed by the late Hacham Ezra Attiah, former Rosh Yeshiva of Yeshivat Porat Yosef in Jerusalem, and this is still the practice observed in the Yeshiva under its current Rosh Yeshiva, Rabbi Shalom Kohen. And, this was the custom of Rabbi Shalom Kohen’s father, the noted Kabbalist Rabbi Efrayim Kohen. Hacham Ben Sion Abba Shaul (1923-1998), also of Yeshivat Porat Yosef, likewise followed this practice.
By contrast, Hacham Yaakob Hillel (contemporary) wrote a responsum demonstrating that it is more proper according to Kabbalistic teaching – and even according to the Arizal – not to say the "Yag Middot" on Yom Tob. And Hacham Ovadia Yosef, in his work Yabia Omer (vol. 9), writes that his personal practice is not to recite the "Yag Middot" on Yom Tob, and that this is the custom followed in his Bet Midrash.
It thus emerges that both customs – to recite the "Yag Middot," and not to recite the "Yag Middot" – are documented in Halachic sources, and both are valid. Therefore, each community should follow its tradition. It is clear that the custom of Halab (Aleppo, Syria), as reflected in our Siddurim, was to recite the "Yag Middot" on Yom Tob, and we should therefore observe this practice as part of our tradition received from previous generations.
Summary: There is some controversy surrounding the custom observed by many to recite the "thirteen attributes of mercy" ("Hashem Hashem Kel Rahum Ve’hanun…") at the time the Torah is removed from the ark on Yom Tob. Each community should follow its tradition in this regard, and the prevalent practice among Syrian Jews is to include this recitation in our Yom Tob prayer service.