We think of summer as long, lazy days, time to relax and have fun. That's because we're not farmers. (And in fact the reason there's summer vacation from school is because, through the early twentieth century, it was assumed that kids would be needed in the summer to work on the farm.)
Medieval people did not think of summer as a time to relax. It was time to get things done. Since probably 90% of the population was engaged in agriculture, this was the time to plow and plant and weed and chase away critters and harvest. It was assumed that during, for example, harvest time (winter wheat, planted at the end of autumn, would be harvested in July) everyone would set to work as soon as it was light enough to see and continue working until it was too dark to see. This is why initially eighteenth-century factories believed in the 18 hour work day ("They work that long in the fields, they can work that long on the factory floor").
Below is a picture of a plowed field. Even with a tractor, it's going to take you a while to work it.
Travel was far, far easier in the summer, in spite of the heat and muddy roads, than it was during the winter, when one was battling cold and snow and ice (and short days). So the kings and great lords who traveled around to different parts of their realms did so primarily in the summer. So did popes--and for the popes, getting out of Rome, which was then prone to malaria, made excellent sense in the summer.
Wars were also fought primarily in the summer. This had always been true--the Bible talks about "May, when kings go to war." An army needs forage for the horses and mules, which means waiting until the grass is growing, and it also prefers that the people they are going to raid have enough food on hand to make raiding worthwhile.
Even though winter was the time for story telling--there were a lot of long cold dark hours to fill--the stories were almost always set in the summer. King Arthur stories typically started with the king at his mid-summer feast, hoping that something marvelous would show up.
Because the most common fabric for clothing was wool, one could get very warm in the summer without the lightweight cottons we take for granted (cotton only reached western Europe in the twelfth century and was expensive--the alternative was linen, also expensive). Men could wear short sleeved tunics that were about knee length, and nothing underneath, but women were expected to wear long skirts, not shorts or mini-skirts. They did tuck their skirts up when working in the field, but they were still more heavily dressed than the men.
Without electricity, no one before the late nineteenth century could have any kind of fan (other than one a human waved), much less air conditioning. Northern Europe was still cool enough for most of the summer that it wouldn't become unbearable, but around the Mediterranean things definitely got toasty.
© C. Dale Brittain 2020
For more on medieval life, see my ebook, Positively Medieval: Life and Society in the Middle Ages. Also available in paperback.