1stSeptember Sikh: Installation of the Guru Granth Sahib in the Harmandir Sahib in Amritsar
The installation of the Guru Granth Sahib in the Harmandir Sahib—the Golden Temple in Amritsar—is one of the most spiritually significant moments in Sikh history. Commemorated annually, this event marks the elevation of the Guru Granth Sahib, the sacred scripture of Sikhism, as the eternal Guru.
https://www.reonline.org.uk/festivals-calendar/installation-of-the-guru-granth-sahib-in-the-harmandir-sahib/
2ndSeptember Zoroastrian: Fravardin Mah Parab
On the day of Fravardin, the 19th day of the month of Fravardin, the first month of the year, Zoroastrians visit the vicinity of the Towers of Silence in India (or in the UK the Zoroastrian Cemetery in Brookwood, Surrey) to participate in a jashan ceremony in memory of the departed fravashis (guardian spirits and souls of the community). Sacred food is prepared as an offering to the departed during the jashan and is later shared by the participants.
https://www.spiritualarts.org.uk/fravardin-mah-parab/
4thSeptember Hindu: Janamashtami / Krishna Jayanti
The Krishna Janmashtami festival marks the birth of Krishna, one of the most popular Gods in the Hindu pantheon.
https://www.reonline.org.uk/festivals-calendar/janmashtami-krishna-jayanti/
8thto 15thSeptember Jain: Paryushan Parva
The most significant festival in Jainism, Parvadhiraj Paryushan Mahaparva, is an 8-day festival of forgiveness and inner cleansing.
https://www.prathaculturalschool.com/post/everything-you-need-to-know-about-jain-paryushana-the-festival-of-forgiveness-and-spiritual-renewal
11thto 13thSeptember Jewish: Rosh Hashanah
Rosh Hashanah marks the Jewish New Year.
https://www.chabad.org/library/article_cdo/aid/4762/jewish/What-Is-Rosh-Hashanah.htm
14thSeptember Hindu: Ganesh Chaturthi
Ganesh Chaturthi celebrates Ganesha as the god of new beginnings, the remover of obstacles, and the god of wisdom, fortune, prosperity and intelligence.
https://www.britannica.com/topic/Ganesh-Chaturthi
15thSeptember Jain: Samvatsari
Samvatsari, often known as International Forgiveness Day, is one of the most profound and spiritually resonant observances in Jainism. It marks the final day of Paryushana, a sacred period of intense reflection, fasting, and spiritual purification.
https://www.reonline.org.uk/festivals-calendar/samvatsari-international-forgiveness-day/
Around 21st September Christian: Harvest Festival/ Harvest Home
Special services are held around this time of year to give thanks for the goodness of God’s gifts in providing a harvest of crops along with all the other fruits of society.
https://www.reonline.org.uk/festivals-calendar/harvest-festival/
21st September Jewish: Yom Kippur
Yom Kippur means Day of Atonement. It is the most sacred and solemn day in the Jewish calendar.
https://www.chabad.org/library/article_cdo/aid/177886/jewish/What-Is-Yom-Kippur.htm
21stto 23rdSeptember Pagan: Autumn Equinox [Mabon / Alban Elued]
Autumn Equinox (also known as Mabon or Harvest Home) is celebrated when day and night are of equal duration before the descent into increasing darkness and is the second festival of the season of harvest.
https://druidry.org/druid-way/teaching-and-practice/druid-festivals/autumn-equinox-alban-elfed
21st to 25th September Buddhist (Japanese): Higan
This celebration marks the autumn equinox for Japanese people. As at the spring equinox, harmony and balance are the themes; sutras are recited and the graves of relatives are visited.
https://www.spiritualarts.org.uk/higan-autumn/
26thSeptember to 2ndOctober Jewish: Sukkot
Sukkot is the Jewish festival of thanksgiving, giving thanks for G-d’s protection when they left Egypt. Also known as the Feast of Tabernacles or Booths.
https://www.reonline.org.uk/festivals-calendar/sukkot-feast-of-tabernacles-or-booths/
With thanks to The Shap Calendar for the valuable information