The 13th day after the vernal equinox is the day of Tištar, the DOG-STAR OF THE THREE-STARS or Sirius, Old Norse Hunda-stjarna.
The day dedicated to Tištar or the most brilliant three stars visible in the firmament is a most auspicious day in the Zoroastrian calendar. The day is known in Iranian folklore as sizdah-bae-dar “13th outdoors” marked by picnicking by the waterfalls and streams.
Vedic Tiṣya (RV V.54.13; X.64.8) corresponds to Avestan Tištar according to the etymological explanation proposed by Forssman (1968), which puts the star Sirius in a clear relationship with the three stars of Orion’s Belt.
The symbolic link with the astral theme of the celestial “arrow” Tyr is strongly present in the Avestan Yasht/hymn to Tištar or the three stars.
The Avestan hymn to Tištar contains two different mythical events. One concerning Tištar’s battle with Ap-aôša “draught” and the latter with the Pairikás, “evil fairies or witches of the bad/difficult year” duž-yaair.
The latter myth with evil fairies and the bad/difficult year corresponds to shooting stars.
In the body of a white horse Tištar attacks Apaôša “draught,” but after three days and nights the three star is defeated, because the three star was not sufficiently worshipped by the Aryans (see Yt. 8.24).
Only after an empowering yasná- “intense wish/desire, zeal” is offered by the Wise Lord Ahûrá Mazdá in favor of the most luminous star (Yt. 8.25,) can Tištar move again against Apaôša and at midday defeat draught ; thus the waters of the Vouru.kaṧa “wide-shored ocean” are free and can be distributed among the seven kingdoms.
Tištar’s epithet is afš-cithra-“having the seed/origin” of the waters (rains), but also “having the brilliance of the waters/rains.”
It is a most lucky day for ancient Iranians.