The Zoroastrian religious calendar

The religious calendar is a matter of some controversy among Zoroastrians.
There are currently three main calendars in use: Fasli, Shahanshahi, and Qadimi. The oldest Zoroastrian calendar ('Old Avestan') was evidently a seasonal one, but it is not clear how it was kept in harmony with the seasons. In later years ('Younger Avesta' and Sasanian times) the prescribed method was to add a month every 120 years, but none of the three modern calendars follow this tradition. Cf. Denkard 3, chapter 419 and Bundahishn chapter 25.

• FASLI ('seasonal'): A religious calendar
  which is kept in harmony with the seasons by intercalculating
  one day every four years, patterned after the Gregorian calendar.
  Naw Ruz (New Year's Day) is fixed on March 21.
• SHAHANSHAHI (or Shenshai): The calendar of many Parsee Zoroastrians, including
  some of the more influential Dasturs (High Priests). It is based
  on a religious calendar which was theoretically synchronized with
  the seasons by intercalculating a month every 120 years, but the
  intercalculation was not consistently followed. Naw Ruz was observed
  on August 23, 1995. A recent proposal would reform the Shahanshahi
  calendar by bringing it back into harmony through the intercalculation
  of whole months.
• QADIMI ('ancient'): In 1746 a group
  of priests and laymen in Surat adopted the Irani calendar on the
  assumption that it represented an older tradition. The Qadimi
  calendar is one month ahead of the Shahanshahi.

Each month and each day of the religious calendar is presided
over by a spiritual being; special prayers (from the Khorda Avesta)
are observed in honor of that spiritual being. Days where the
same spiritual being presides over both month and day (marked
with * in the table below) are particularly sacred.

See also Adarbad Mahraspandan's advice
on activities for each day of the month.

Names of the days and corresponding dates of observance per
Fasli calculations:

Legend:

* = Name day feasts

r = Rapithwina begins Mar 23 ends Oct 16

s = Festival of Sadeh (Jashan-e Sadeh)

g = Gahambars ('feasts'):

1992-5 1996-9 2005-8
Maidyozarem ('mid-spring' feast): 
F: Apr30-May4 Apr30-May4 Apr30-May4
S: Oct2-6 Oct1-5 Sep29-Oct3
Q: Sep2-6 Sep1-5 Aug30-Sep3

Maidyoshahem ('mid-summer' feast): 
F: Jun29-Jul3 Jun29-Jul3 Jun29-Jul3
S: Dec1-5 Nov30-Dec4 Nov28-Dec2
Q: Nov1-5 Oct31-Nov4 Oct29-Nov2

Paitishahem (feast of 'bringing in the harvest'): 
F: Sep12-16 Sep12-16 Sep12-16
S: Feb15-19 Feb14-18 Feb11-15
Q: Jan16-20 Jan15-19 Jan12-16

Ayathrem ('bringing home the herds'): 
F: Oct12-16 Oct12-16 Oct12-16
S: Mar15-19 Mar14-18 Mar13-17
Q: Feb15-19 Feb14-18 Feb11-15

Maidyarem ('mid-year'/winter feast): 
F: Dec31-Jan4 Dec31-Jan4 Dec31-Jan4
S: Jun4-8 Jun3-7 Jun1-5
Q: May5-9 May4-8 May2-6

Hamaspathmaidyem ('coming of the whole group [of farohars]', feast of All Souls): 
F: Mar16-20 Mar16-20 Mar16-20
S: Aug18-22 Aug17-21 Aug15-19
Q: Jul19-23 Jul18-22 Jul16-20
- Ahunawad
- Ushtawad
- Spentomad
- Wohukhshathra
- Wahishtoisht
(Parsi Mukhtad adds 5 days to beginning)

Noruz ('New Year's Day') 
F: Mar21 Mar21 Mar21
S: Aug23 Aug22 Aug20
Q: Jul24 Jul23 Jul21

Watches (gahs) of the day:

Many prayers are altered according to the time of day.

• Hawan (sunrise to noon)
• Rapithwin or Second Hawan (noon to 3 p.m.)
• Uzerin (3 p.m. to sunset)
• Aiwisruthrem (sunset to midnight)
• Ushahin (midnight to sunrise)

Important festivals

• Gahambars
• Maidyozarem ('mid-spring' feast)
• Maidyoshahem ('mid-summer' feast)
• Paitishahem (feast of 'bringing in the harvest')
• Ayathrem ('bringing home the herds')
• Maidyarem ('mid-year'/winter feast)
• Hamaspathmaidyem (feast of 'All Souls')
• Noruz (New Years)
• Jashan-e Mihragan
• Jashan-e Tiragan
• Jashan-e Sadeh

Significance of the names

The first seven days celebrate Ahura Mazda and the Amesha Spentas, (lit. Beneficent Immortals) sometimes called the Archangels. They are the highest spiritual beings created by Ahura Mazda.

The second week celebrates LIGHT and NATURE:

The third week celebrates moral qualities:

The fourth week celebrates religious ideas:

From www.avesta.org