Gemara:

Rosh Hashana 

Halacha:

Shabbat,  Tefillah 

Holidays:

Aseret Y'mei Teshuvah 

Parsha:

Haazinu 

References: Rosh Hashana: 31A  

Description

Why does every other Yom Tov have a [optional/custom] psalm to say in honor of it [such as ברכי נפשי on Rosh Chodesh] that the Leviim used to say on the additional sacrafices in honor of that day [i.e.-Musaf] yet on Shabbat, when they would say portions from פרשת האזינו on the Musaf-sacrafice, NOBODY has the custom to say it?