” Nome (dal medio persiano apastāg, forse «testo fondamentale») del complesso dei libri sacri della religione di Zarathustra (➔ zoroastrismo). I libri sono da distinguere dai commenti (zend, «esegesi») sorti intorno a essi e redatti in pahlavico. La redazione dell’A. che attualmente possediamo risale all’età sasanide e si compone di 5 parti: Yasna («preghiera»), Visprat o Vispered («tutti i capi», invocazioni e formule d’integrazione del precedente), Vidēvdād o Vendidād («legge contro i demoni», prescrizioni per purezza rituale, pene ecclesiastiche e norme giuridiche), Yasht («adorazione, sacrificio», 21 inni) e Khvartag apastāg o Khorda A., («piccolo A.» libro di preghiere, per i laici). Secondo la tradizione l’A. sarebbe stato scritto in lettere d’oro su 12.000 pelli di bue da un discepolo di Zarathustra, Kavi Vishtaspa, o da Giamaspa, ministro di Vishtaspa. La redazione definitiva avvenne verosimilmente sotto Sapore II (309-379).” (tratto da Treccani.it)
“The Avesta that has survived destruction is organized in different ways.
System One – Five Sections
One system organizes them as five books and miscellaneous fragments – the manner in which they are published in print. And this can vary from one edition to the next, especially in the composition of the Khordeh Avesta selections. The five books are the Yasna (including the Gathas), Visperad, Vendidad, Yashts and Khordeh Avesta.
System Two – Two Sections
Another system divides the Avesta into two sections:
The first section consists of the Vendidad, Visperad and Yasna – texts used in priestly liturgies of the inner circle. The manuscripts are found in two different forms: separately, each with a Pahlavi translation, and together, with chapters of each book intermingled according to the requirements of a particular liturgy. The latter form does not contain Pahlavi translations and is therefore called the Vendidad Sadah (plain).
The second section consists of the Khordeh Avesta – texts for liturgies of the outer circle and those used by both, priest and laity in their daily prayers and activities. Under this system, the balance of the texts not found in the first section are found in this section – texts including the Yashts.
Yasna (also spelt izeshne in later texts) means service, prayers and dedications – the words of worship (cf. Sanskrit yajna and yana). Priests recite the Yasna as part of the liturgy when performing their priestly duties and functions. The gathas or hymns of Zarathushtra are part of the book of Yasna.
The Visperad is recited as part of the liturgy used to solemnize Gahambars (seasonal gatherings and feasts) and Nowruz (New Year’s Day). The Visperad is always recited with the Yasna.
The name Vendidad is a later form of Videvdat, which is in turn a contraction of Vi-Daevo-Data, the law against devas or evil. They are the prayers used by priests in purification ceremonies. The Vendidad is also a store of Zoroastrian history.
The Yasht (yasht is commonly translated as worship) are hymns dedicated to Zoroastrian ideals together with the related angel (such as the ideals of friendship, the word as bond, and kindness, and the guardian of these ideals – the angel Mithra), and Zoroastrian concepts such as the fravashi. Originally there were thirty Yashts, one dedicated to each named day of the month. Today only twenty one survive. The Yashts are sometimes recited by the laity as prayers on the named day of the month or on an specific occasion (for instance the Farvardin Yasht, dedicated to the Fravashis, is recited during memorials).
The Khordeh Avesta, or concise Avesta, are selections of the Avesta used by the laity for daily prayers or when performing certain tasks. The Yashts, either in their entirety or selected Yashts, are sometimes included in the Khordeh Avesta since both are recited as prayers by the laity. Included in the Khordeh Avesta are the baj prayers said before undertaking a task. The baj are also prayers said for the dead on the occasion of a death anniversary. The baj can be recited before eating or drinking when it can be compared to grace said by Christians.
Also included in the Khordeh Avesta are the niayesh which in Persian means prayers – prayers recited in conjunction with gahs. The gahs, in turn, are prayers for the five periods of the day.
Included in the Avestan fragments are the afringans which are liturgies of the outer circle – blessings offered during a Jashne / Jashan (thanksgiving) ceremony, and the Siroozeh or Sirozah (also called Siroja Yasht in India), short dedications to the thirty days of the month.
As mentioned above, the gahs are prayers for the five periods of the day. ” (tratto da http://www.heritageinstitute.com/zoroastrianism/scriptures/#contents)