Near the autumnal equinox, many people within the pagan community celebrate the harvest. Many people celebrate Mabon at this time. The Filianic community (and others of related faiths) celebrate Cuivanya. And there are people within the pagan community who don't celebrate this time of year beyond the secular joys of football and pumpkin spice everything. The 21st of September is generally the day these celebrations happen on (though the equinox falls on the 23rd this year). It falls roughly at the mid point of the harvest season in the northern hemisphere. This season is reflected in the names of the moons for the months of August, September, and October.
The full moon of August is known colloquially as the full Corn moon. This is because the grains planted in spring begin to become ripe at this time. August first is the date of Freyfaxi, Lammas, and Chelanya. Freyfaxi is a holy day for Freyr, wherein the first harvest is celebrated, his sacrifice for the good of all life is recalled, and (for me) a day honoring the labor of taking in the harvest. Lammas has many spellings. The date is also called Lughnassadh, in honor of the Irish Celtic god Lugh whose myth cycle includes matters pertaining to grain, fertility, and prosperity. The Wiccan community (and pagan faiths derived from Wicca) honor Lammas as a time of fertility and prosperity. There is much celebration of things that have come to fruition over the year. This is the theme over the entire harvest season.
Chelanya is the first of the Filianic harvest connected holy days. It is, by some, considered to be a mirror of the celebration of Eastre, when the Daughter dies and then returns from life. Chelanya is a time focused upon the resurrection of the Daughter and the soul into new life as children of Dea. The time of Chelanya is a period where the resurrection/rebirth of the world that comes with the Daughter's return from death is celebrated along side the harvest. Scriptural readings focus upon the elements of the Daughter's mythos where she arises from death and upon her giving of herself in holy sacrifice1.
The full moon of September is known colloquially as the full Harvest moon. During this time, the harvest is in full swing and pretty much everything that is ripe this time of year is ready for gathering. Mabon is a celebration of, amongst other things, the grape harvest. This is because the grapes become ripe for harvest at this point and traditionally wine making began with the harvest. Wine, in the Wiccan faith, is considered to be holy and by some to be the blood of the Green Man (also known as the Harvest God, a vegetal deity who, again, sustains the world through his death and rebirth).
Cuivanya is the second of the Filianic harvest connected holy days. It is the celebration of Divine Life. The focus of this holy day shifts from the renewal of all things in the Daughter to the life of Dea in all three aspects. The predominant focus rests upon the Bright Mother. The bounty of the season is celebrated as gifts from Marya to the world, given to keep all things alive and well. Interestingly, where the symbol most predominant with Chelanya is the grain itself, the symbol that is most important in Cuivanya is the sickle. Here, we see Dea as the giver of life in the sickle that cuts the grain which feeds us. We also see Dea as the Dark Mother to whom all life returns in the sickle as it ends the life of the grain2. Another symbol of this holiday is the apple of wisdom, due to this being when apples are most predominant. The common meditative focus of this holy day is Dea as the Ground of All being, the source of all that exists.
The full moon of October is known as the full Blood Moon. In antiquity, this was the time when livestock was slaughtered in preparation for the winter. This could be described as the final harvest of the season when the herds are culled to both provide meat for the winter and to render them a size that could survive the winter given the supplies set by for the coming season. At the end of October comes the celebration of Halloween, Samhain, Winter Nights3, and Tamala. So much has been written about Halloween and Samhain, I really leave that to others right now. Winter Nights is when a blóts are held to honor the alfar and the disir. Many focus upon honoring the dead at large at this time. In antiquity, this celebration was held during the first three days of winter.
Like Winter Nights, Tamala is a three day celebration. It is the third of the Fire Festivals (the other two being Sai Herthe's Day in January and Rosa Mundi in June). Tamala is the time where Dea as the Dark Mother is honored. But it is predominantly when the dead are honored. Tamala is when death is a meditative focus. Here, it is not the cosmic death of the Daughter or the 'death' of the universe as it is brought into union with Deam Mysterium. The death that is considered is the death of the individual and the transmigration of the soul through the lives that happen upon the wheel of incarnation. It is a time where the living reach out to the dead and honor them. It is also a time where the 'Family of the Faithful' is celebrated, wherein all are united within the Daughter via her sacrifice. Thus, there is emphasis placed upon reunion with those who have died before us and shall be reborn into incarnation at some point in the future.
Now, back to the matter of September's celebrations. I personally view Cuivanya and Mabon as much the same celebration. Both are harvest celebrations that focus upon life's preciousness. I also consider this time to be one where the holy work of harvesting should be honored, though I primarily celebrate the work of harvest at Chelanya/Frefaxi/Lammas. If I am able, I give offerings of what I have harvested thus far through the season. I also try to make a point at this time of year to begin the process of preparing for winter. If I have nothing I have harvested (such as produce or herbs), I try to give an offering of homemade bread or something else that I have fashioned myself.
If you celebrate the Filianic holiday or the Wiccan/Wiccan derived one, it is still a time for thanksgiving and preparation for the coming season.
1. The sacrifice of the Daughter is much like the sacrifice of Freyr. Her life is given for the sake of the world and it is compared to the lifecycle of grain. While orthodox Filianism does not give much exposition upon this, it is my belief that the Daughter's sacrifice not only brings the rebirth/resurrection/renewal of the world but also that which sustains it. For, like Freyr, she sustains the world through her actions.
2. While the grain is technically not alive anymore when it is harvested, it is viewed as still 'alive' while it is standing in the fields. This is an ancient perspective that is reflected in some of the oldest of folk music out there. I encourage you to consider the song John Barelycorn as a British isles manifestation of this musical theme.
3. Winter Nights is celebrated pretty much anytime between the Blood Moon and Yule in the Heathen community. Many of the kindreds that I know of will celebrate it around the time of Halloween because it is convenient and dovetails nicely with the secular celebrations of this date.