Harvest Month(28 days, 4 weeks, similar to August, Full Moon is on the 14th)
Harvest Month is when troops return to their homes from summer campaigns to shore up defenses before the goblinoid raids make camping too dangerous. The peasant classes are now in full swing, finishing the last of the hay harvest and starting in on the harvesting of other major crops.
Goblinoid tribes start fighting, often against other tribes or traditional enemies(Goblins vs. Orcs, Kobolds become even more belligerent against gnomes, etc) but they will not turn down easy targets of other races.
Their raids tend to be much more reckless(+3 bonus to the morale of Orcs, Goblins, Kobolds, Hobgoblins, and Bugbears during this time), but instead of killing, they will take captives, even those of races they normally don't spare. Those captured will be tortured until the new moon just prior to the Harvest festival. The favored captives of the goblinoids are children.
Autumn Month(28 days, 4 weeks, similar to September, Full Moon is on the 14th)
Autumn Month is when most humans(even a few nobles and most soldiers) are involved with storing and counting the harvest. Those not involved with that are making war against the humanoids to defend their settlements.
The goblinoids have massively stepped up their raids, raiding literally anyone(+6 bonus to morale). The victims are tortured until the New Moon(the 28th) when they are all slain in a mass sacrifice dedicated to their racial gods as well as to Stalker(the goblinoid bogeyman/grim reaper). The goblinoids take the heads of these sacrifices, cover them in pitch, present them in varying manners outside their settlements(hang them from trees, impale them on spikes, leave them in piles, etc), and set them ablaze each night in an attempt to keep stalker away.
Harvest Festival(7 days, 1 week, Halloween/Oktoberfest/Thanksgiving)
Those of strong faith in the Imperial religion spend most of the festival in prayer. For others, this is a festival of plenty, to celebrate the finish of the majority of the harvest.
A tradition that started a few centuries ago, initially among settlers from the north(it's said there's a religious significance to the practice) has spread among the various settlements, regardless of origin. Children carve hideous faces into squashes, gourds, and pumpkins. Each night, these jack-o'-lanterns are placed around the homes and on the walls of the settlements, with candles inside them. The more jack-o'-lanterns a settlement has, the fewer goblinoid attacks the settlements seem to suffer during the nights of the festival. Many children also use this time to perform pranks(and blame them on goblins) while the adults tell scary stories of goblins, witches, and the undead.
Wine Month(28 days, 4 weeks, similar to October, Full Moon on the 7th)
The various human settlements are involved with finishing the late plowing, and finishing the storing of the harvest. In areas with vineyards and orchards, this is when orchard crops are finished being gathered and beverages requiring year long fermentation or aging are bottled.
Goblinoid raids continue throughout this time, as they seek more heads to burn each night. By this point the number of heads so greatly outnumbers the number of jack-o'-lanterns a community can make, that tactic no longer works for the humans, and is thus abandoned in favor of massive defenses.
As if the goblins aren't bad enough, the people of Midlands have two more problem to deal with; the start of Wyvern breeding season. Unlike the younger wyverns that plague them during the spring, the Wyvern issue in the late fall and early winter is that fully grown male wyverns are on the move, leaving normal territories to seek mates.
The other problem is Ankhegs. Like the Wyverns, this is their breeding season. It's also when they're seeking to eat enough food before the first winter snows. Males tend to be less aggressive until the end of breeding season(if they're unsuccessful in finding a mate), while females tend to be extremely aggressive, not only to fatten themselves up, but to create a pile of carrion for their young to feed upon over the winter.