The order of prayer on Chol Hamoed, 3 Seifim: 1. On Hol Hamoed, we take out a Torah scroll and read four sections about the sacrifices of the holiday from Pinchas. On the first day of Hol Hamoed, the first reading is "On the second day," the second reading is "On the third day," the third reading is "On the third day" again, and the fourth reading, reflecting the doubtfulness of the day, is "On the second day...And on the third day." We read this way all the other days. Rem"a: There are those who say that the first two read the doubtful day portions, the third reads tomorrow's section, and the fourth goes back and reads both doubtful day portions that were read by the first two readers, and this is our practice. On the seventh day, the first reading is "On the fifth day," the second is "On the sixth day," The third is "On the "On the seventh day," and the fourth is "On the sixth day...and on the seventh day." This is our practice (Rash"i in the name of his teachers, Mahari"v, Minhagim). In Israel, where there is no doubt about the day, they only read the sacrifice of the day. On the second day, which is the first day of Hol Hamoed, the first reading is "On the second day" and the three next readers read the same section. This is done for the rest of the days.
2. On Shabbat of Hol Hamoed, Arvit, Shacharit, and Mincha are like on Shabbat with the addition of "Yaale v'yavo" in the Avodah section. In Mussaf, we say "You have chosen us," "Because of our sins...", "this holiday of Sukkot and this day of pleasure," finishing with "who sanctifies Shabbat and Israel and Holidays." Rem"a: The practice is to read Ecclesiastes on the Shabbat of Hol Hamoed or on Shmini Atzert if it falls on Shabbat (Mahari"l). See 490.
3. We take out two Torah [scrolls]. From the first, we read "Look, you have been telling me..." [Exodus 33:12] and the maftir reads from the sacrifices corresponding to the possible day. The haftarah is from Ezekiel, "It will come to pass on the day that Gog comes..." [Ezekiel 38:18].