Shulchan Aruch writes in Siman (section) Kuf (100) that when a person is ready to pray on special days of the year or of the month, whether it be holidays or Rosh Chodesh once a month, in preparation for those special days, he must review the tefillah (prayer) ahead of time and go over the different parts that need to be added. In particular, when you have Tefillat Musaf (additional prayer), which is different from the regular tefillah that we pray three times a day, the person should look it over, open up the siddur (prayer book), and review it to make sure that it's shegura b'fiv, that the person has fluency in those tefillot, and that they're going to pray them properly in advance of those special days.
The Rema, Rabbi Moshe Isserles, qualifies and says that this was only once upon a time, when they used to have to pray by heart. But now that we pray from the siddur and have the siddur and the tefillah laid out in front of us, this would not be as much of a mandatory requirement as it was once upon a time. Nevertheless, the Mishnah Berurah and many of the Acharonim (later authorities) say that one should not be lenient on this matter. A person should still review it ahead of time, even with the siddur, so that they know how to read it properly, have proper intent, and understand the prayers they're going to be offering. They emphasize this especially when it comes to Yamim Nora'im (Days of Awe). But even when it comes to Rosh Chodesh, which is only once every 30 days, they say that a person should make sure to review it, especially if there are piyutim and poems and things that he might not know what they mean. In order for a person to understand the tefillah, they should review it ahead of time.
One nice point the Mishnah Berurah makes is that a person should not say Birkat HaLevana, which is only a beracha (blessing) once every 30 days, once a month. He should not say it by heart without looking into the siddur, even if he thinks he knows it, because he hasn't reviewed it, unless he reviews it ahead of time. Some wanted to say that since other people are saying it together with him, he could follow along with them. Nevertheless, any time a person is saying something that they're not saying so frequently, they should review it ahead of time to know how to say it and what it means.