Heathenry and Autumn
Heathens, from Germany to Scandinavia, did not have a centralized holiday for the Autumn Equinox, as with so many other Neo-Pagan holidays. This is one of the reasons being a traditionalist in the world of New Age is increasingly difficult, but I feel the past is the best teacher for moving forward. Seeing as they only distinguished two seasons, summer and winter, and summer was extremely busy, we often see this long gap in sacrifice and Blots.
Most of the time, this was a rest after a very long pillaging and planting season. They could finally reap the rewards of their difficult early summer season. However, when and how many vary from peoples to peoples (the Germanic are a VAST people) usually anywhere from Mid-August to Mid-October as it depended greatly on their environment. The English and German's traditions have managed to stay somewhat intact and are still largely celebrated, though Christianized. The English have two; a wheat and an apple harvest, and the Germans usually celebrate theirs the last day of September or the first Sunday of October. It was likely moved to the 21-22 when the Gregorian calendar was adapted, but before then is unknown as far a specific date. Both coincide with the Anlgo-Saxon Haerfest.
Now, there is the Egil's Saga(1) where it mentions Haustblot in later August and roughly translates to Fall Season Day. However, I would doubt that because is speculation in the author being none other than Snorri, and he is known for getting things wrong (I.E. Eostre). There are other references to leaving Sleipnir the last piece of your crops when harvesting in a way of giving back your thanks to Odinn in Saxony. Though there is some discussion to this relates more to the Winter Nights than Fall.
How They Celebrated
In the beginning, they would collect the wheat and other grains. It's likely they gave thanks to Sif especially at this time because her hair was golden like the wheat field and she is likely the goddess of vegetation in general. In the story where Loki cut it, we could see it being portrayed in the Harvest and then giving some back to her with the intent of her wig. This can also be further dived into throug her husband, Thor, since in the beginning of the Germanic Heathenry, Thor was actually the god of air, itself, which brought about the storms and rains. (2)
Norse Goddess Sif (artist unknown from pinterest)
Norse Goddess Gefjun and her four sons as bulls (art from unknown artist on pinterest
Gefjon is another of the disir that likely would've been praised in the way of giving back. She's often called the "Generous One" and is well known during the planting season as she plowed the entire country of Denmark (?) and brought their Zealand Island out of the sea. In this case, it would make since that they give back to the Earth in her name by fertilizing the ground in preparation for next years harvest. This is, again, a common theme that is continued on into the Winter celebrations. I would further this by her mention in the part Lokasenna where Odin interupts and basically says not to anger Gefjon because she either knows the fate of man or has some kind of hand in it. This little tribute before and after could be a way of pleasing her for a better life, or at least another year. (3)
Freyr and Gullinbursti http://www.howarddavidjohnson.com/nordicmyths.htm
Another God who is popular at this time would be Freyr, or Ing to the Anglo-Saxons, the brother of Freyja, who is actually another Sky God. He's seen as the other half of Thor in making the crops by ruling over Sunshine and actual Rainfall. This is where we see the two sides, Vanir and Aesir, really working hand in hand. He was rarely hated, as said in the Eddas, and most likely because him liking you directly related to your prosperity and abundance. He was paraded around in chariot, just as his mother Nerthus, in early Germany. Later in the Norse Heathenry, he was instead celebrated by horse races and a horse was often sacrificed in his honor at the end of the games.
Idunn and her magical apple of eternal life were also a common theme, but closer to where we now see the Neo-Pagan Samhain. It's very possible that as they collected the apples and made their mead and other drinks, that they have her praise since this is really all we know about her. Odinn also is a common theme, but again, likely around this time as it was much closer to Winter.
Credit to these stories: