This is the second book of the Mishneh Torah of the Rambam, Rabbi Moshe ben Maimon. Laws of Tefillah.
This chapter explains who instituted the blessing over heretics and why, introduces the "Havineinu" prayer, who says it and when, and orders the prayers for Shabbat, Yom Tov, Rosh Hashanah, Yom Kippur, Yom Kippur in the Year of Juvel, Rosh Chodesh, Chol HaMoed, and Yom Tov that falls on Shabbat and after Shabbat, Hanukkah, and Purim. It also addresses whether "For Miracles" is mentioned on Hanukkah that falls on Shabbat, the fasting individual who mentions "Answer Us" in "Shomea Tefila," and the congregation's messenger who recites it between "Redeemer" and "Healer." Finally, it covers the order of the 9th of Av, when we begin to pray for rain, and the order of the prayers for the Ten Days of Repentance.