Some Interfaith September Inspirations:

Native September
For Native Americans, September marks the season of Genuuqwiikw, the season of mountain trails and the beginning of the fall hunt for game; the Iroquois Green Corn Ceremony, a time of renewal involving dances, fasting, offerings, and readings from the code of Handsome Lake;
and the Jicarilla Apache Ghost Dance in New Mexico.

Hindu Ganesh Chaturthi Sept. 6/7
A Hindu festival honoring the god of prosperity, prudence and success.
Images of Ganesha are worshipped.

Paryushana Parva ~ August 31st ~ September 7
 Jain 8 day festival signifying human emergence into a new world of spiritual and moral refinement.
Marked by recitations from Jain sacred writing and family exchange of cards and letters.
Celebration of the natural qualities of the soul. The 8th day (Samvatsari) is most important and is focused on forgiveness; a solemn occasion for examining one’s thoughts and feelings, and for asking forgiveness for offenses committed against others through deeds, words, or thoughts.

Baha'i ‘Izzat ~ September 7  
The beginning of the tenth month of the Bahá’í year, meaning “might.”

International Literacy Day ~ September 8
Since 1967, September 8th has been designated as a day committed to calling attention to the importance of literacy and acting as a call to action for universal literacy.

Christian Nativity of Mary ~ September 8
Christian celebration of birth of the Virgin Mary.

(US) National Grandparents Day ~ September 8 ~
In 1978, then President Jimmy Carter declared that the first Sunday after Labor Day,
would be Grandparents Day! ~ Celebrating the lives and influences of grandparents.

New Thought Christian Unity World Day Of Prayer ~ September 11/12
31st annual, worldwide 24 hour Prayer Vigil.
This year's theme is Moving Mountains and the affirmtion is,
Fueled by faith, I can move mountains.

Ethiopian New Year (Rastafari ) ~ September 11
The start of the New Year in Ethiopia is recognized because Rastafarians believe Ethiopia
to be their spiritual homeland, and a place to which they want to return. The Ethiopian calendar
 is the principal calendar used in Ethiopia and also serves as the liturgical calendar for Christians
in Eritrea and Ethiopia belonging to the Orthodox Tewahedo churches, Eastern Catholic Church
and Coptic Orthodox Church in Egypt.

Zoroastrianism Ghambar Paitishahem begins ~
September 12 Runs through Saturday, September 16
This festival celebrates the creation of the earth and the summer harvest.
Paitishahem literally means 'bringing in the harvest'.

Christian Elevation or Exultation of the Holy Cross ~ Sept 14
This day recognizes the Cross as a symbol of Christ’s love for humankind and God’s victory over death.  It also marks the finding of the Holy Cross by St. Helen after it had been stolen in the 7th century C.E.  Orthodox churches begin their commemoration at sundown on the preceding day.
In the Ethiopian Orthodox tradition, this day is known as Meskel and is marked on September 27th. Followers are invited to lay down their own burdens or crosses they bear.

Mawlid al-Nabi ~ September 15/16
Mawlid is an annual festival and holiday commemorating the birthday of Muhammad on the traditional date of 12 Rabi al-Awwal, the third month of the Islamic calendar. A day central to the Sufi tradition of Sunni Islam, the Mawlid is also celebrated by Shia Muslims. From: Wikipedia

JainismHinduism Anant Chaturdashi ~ September 17
For Hindus this day celebrates Anant, one form of Lord Vishnu. If a devout Hindu vows to honor Vishnu and keeps that vow for 14 years, it is believed that she or he will see wealth. This is also the last day of Ganesh Utsav, a 10-day Hindu celebration. Statues of Lord Ganesha are carried to bodies of water to be submerged, accompanied by devotional songs and dancing.  Jains celebrate this day with special worship services and processions to the community’s main temple.

Kshamavani (Sanskrit: क्षमावाणी) Forgiveness Day ~ September 18
is a day of forgiving and seeking forgiveness for the followers of Jainism

Wicca/PaganOstara & Mabon September 22
(northern/southern hemispheres, begins sundown 21st)
The time when the sun is directly above a point on the equator.
The equinox will be either Vernal or Autumnal depending on the hemisphere.
Equinox celebrations as days of equal light & dark mark the turning of the wheel.

Mabon: Wicca observance of the autumnal equniox when day and night are of equal length.
A harvest festival time.
Ostara: Wicca welcoming of spring and the goddess-as-maiden.

Shinto Shuki-sorei-sai September 22
A memorial service similar to the March equinox service (Shunki-sorei-sai), this day is marked by the cleaning and purification of gravesites and the reverence of ancestors as kami, or divine spirits.

Buddhist Ohigon September 22
A celebration of the equinox that is of particular importance to Japanese, Korean, and Tibetan Buddhists.  During this festival, the six Paramitas [virtues] are emphasized:
generosity, morality, wisdom, honesty, endeavor, and patience. 

Native American Autumn Feast September 22
A day to honor the harvest end and the coming and going of the seasons,
including prayers, songs, and the telling of tribal stories. Native tribes in the U.S. Southwest, including Hopi and Navajo, celebrate equinox with autumn vision quests. 

Baha'i Mashí’yyat ~ September 26 
The beginning of the eleventh month of the Bahá’í year, meaning “will.”

Native American Day ~ September 27
Celebrates Native American history and culture, the fourth Friday of every September.

Christian Arch Angel Michael & all Angels Day ~ September 29
This is the feast day of St. Michael and all the Angels. It is the most ancient of all the angel festivals.The Anglican church celebrates all angels, both named and unnamed on one day.
Roman and Orthodox Churches separate them into two categories (with the unnamed angels
having their feast day on October 2nd). From fairly early on, Michaelmas was an important holiday,
the religious or Christian equivalent of the autumn equinox. Called Michaelmas originally beacuse each of the arch angels had their own day, now generally all celebrated on this day.