The name of the tenth month of the Zoroastrian calendar is DAY. It is the happiest month of the calendar. It starts on December 16 and ends about January 14. The name DAY comes from the Avestan daδváv/daðváv “creator.”
Daδváv/daðváv is an epithet of Ahûrá Mazdá that became the name of the 10th month, as well as of the eighth, fifteenth, and twenty-third days in each month of the Zoroastrian calendar. Avestan daδváv/daðváv comes from the root dá “to give, gift, put, place, create, DO.” The root dá can be compared with Old High German tuon, German tun, English to do; “to give, gift, create.” In the last sense it is used for the creative acts of Ahûrá Mazdá.
For example in the 3rd rhymed verse line of Yasna 44.3 of the poetic gathas, the prophet asks; “Who created (dát) the path(s) of the sun and the stars?” (Compare dát with Old Church Slavonic dati, Lithuanian dúoti “to give.”)
Another example would be the 5th rhymed verse line of Yasna 44.7, where Manzdá (genius, mind-power) is invoked as the “creator of everything” (vîspa.nạm dátárəm) through his splendid, auspicious spirit/mind-energy (spéñtá mainyü.)
Avestan daδváv/daðváv “giver, creator” is comparable to that of vîδváv/vîðváv “knowing, wise.
Also the most common invocation formula of Vendidad is “O Ahûrá Mazdá . . . creator (dátaré) of the material world . . .
In Old Persian texts, Darius the Great states: “. . . Ahûramazdá who created (adá) this earth, who created yonder heaven.”
In Pahlavi or middle Iranian commentaries of Avestan texts daδváv/daðváv is always glossed with dádár.
The tenth month was named daθušö/dathušö (māh-month, moon) of the creator.” The first day of each month was called ahûrahæ mazda (day) of Ahûrá Mazdá, and the eighth, fifteenth, and twenty-third days also bore the epithet daθušö/dathušö Giver/Creator of “heat, fervor, fire “áthrá;” friendship, love “mithrá” and power to see, vision “daæná.”
According to the Bûn-dahišn (30.10-31.8, 33.15-34.2), the establishment of the names of the month after “splendid, auspicious immortals” was an essential part of the material creation, especially the creation of finite time, which places a limit on the life span of Ahriman or the beaten, afflicted spirit.
As participants in the battle against Ahriman the three Days function not only as mere day names but also as Öhrmazd’s coworkers, designated as “space” (gáh), “foresight, vision” (daæn), and “time” (zamán).
In the Old Persian Achaemenid calendar, the 10th month is called a-náma.ka– “unknown or undiscovered name” referring to Ahûrá Mazdá, (spirit/mind power, genius.)