The most crucial labor in August was harvesting and threshing the rye and winter wheat, which would have to sustain the community through many months to come.
August 1st began the feast of Lammas, when the first grain of the new harvest was consecrated and made into bread for the Eucharist. There would be more harvest celebrations yet to come in September and October, but Lammas celebrated its beginning, and was as much a symbol of the spiritual harvest as it was the physical.
Wheat was cut down with a sickle; a short handled tool with a curved blade. Those reaping the wheat would grasp a handful of it, well below the heads of grain, and cut the stems. They would be bundled into small sheaves at first, then into larger ones called shocks that could be left standing in the sun to dry out. Once all of the grain had been harvested from the field, first the poor could come through and glean any fallen grain, then the livestock would be allowed to eat down the stalks and forage.
Long stretches of dry weather were extremely important for a good harvest. Too much damp weather could ruin the grain, or leave it tainted with ergot, a dangerous, toxic fungus that could do severe physical and neurological damage, causing hallucinations and paranoia. There has even been some speculation by historians correlating the most intense periods of witch hunting and inquisitions, with the worst outbreaks of ergot tainted food.
If grain could not be dried out in the sun, due to unseasonable weather or climates with damp or short growing seasons, grain dying kilns might be used. These were shaped much like regular medieval ovens, but larger, with well-constructed flues.
Once the grain stalks were dried out, the next task would be threshing and winnowing. This process separated the useful heads of grain, which could be made into flour, from the useless stalks and chaff that could not be eaten.
A flail was used to beat the wheat bundles, which shook loose the grain. If done outdoors, the chaff would naturally blow away on the wind—but it might also take some of the grain with it. Therefore, threshing was often done inside barns. This made it an activity that could continue through the fall, and even winter months if necessary.
I found this very short video that shows someone using a flail to separate grain from wheat stalks. There is even a portion of it where you can actually see the wind lifting away the chaff. The hand-crank machine he uses at the end is obviously not period, but everything before that is informative for those who want to see how this was done. Can you imagine doing this kind of labor for hours at a time, days on end? I would imagine it was a sweaty, back-breaking task in the August heat—with no possibility of a soothing shower at the end of the day!