The most festive time of the year in the Zoroastrian religious calendar is the 10th month called “day” in modern Persian. In the Seasonal Avestan calendar, the tenth month starts around mid-December and ends around mid-January.
The festivals in the month of day “creator,” fall on December 16, December 20th -21st, December 23, December 30th-January 4th, and January 7th.
In the Zoroastrian calendar, the first day of each month is dedicated to the supreme god ahûrá mazdá, while other days of the month are dedicated to each of his Immortals. When the name of the month and day(s) coincide there is a great celebration/festivity.
One can tell a lot about a people by their rituals, symbols and festivities. The festivities celebrated by the ancient Zoroastrians symbolized the infinity and beyond, and the sacred will to bring the creative brilliance of the Immortals to us. On this month of day, the god powers of mind/imagination, heat/passion, love, and vision to create are celebrated.
Day, the name of the 10th month goes back to the Avestan daðváv, and comes from the Proto Indo European root *dheh “to put, set, establish, do, create,” (See Didier Calin, Dictionary of Indo European Poetic and Religious Themes pp-179.)
*Dheh refers to a creation, order/design that is established by the Immortal Gods, and has a particular SACRED connotation.
On December 16, the great god ahûrá mazdá, who creates through the wondrous powers of his MIND, is celebrated. For Mazdá is the “power of mind/creative imagination to manifest itself, and to do, create.” Mazdá is a cognate of Muses and goes back to Proto Indo European *mens dheh.
On December 20th -21st during winter solstice, the triumph of the light imperishable, the creative energies of the invincible sun are celebrated.
On December 23, sacred flames/fires of hearth and altar (átharš) referring to the will power, heat and passion as soul/spirit of the creation are celebrated.
On December 30th or 31st, the inspiring creativity that comes from love (mithra,) bond, kinship with the gods, is celebrated.
On January 7th the creative power of “vision, imagination” (daæná) is celebrated.
The second most important Zoroastrian thanksgiving holiday after the Spring Equinox called maið-yaar “mid-year,” begins on December 31 and ends on January 4th.
Maið-yaar refers to the middle or heart of winter, for in ancient times, the years were counted by the passage of winters.
Also on January 24, cadæ the great festival of “discovery of fire in the middle of the freezing winter” is celebrated.
In conclusion, I shall add that in the Old Persian/Achaemenid calendar, the 10th month (December-January) was called A-NĀMA-KA. The term a-náma-ka means literally “un-named, full of mystery, unrevealed,” and referred to the all gifted, god of wisdom, vision and superb mind-powers Mazdá, and his God Powers of Creation that are specifically hallowed during this festive month.