Feres or fers (roughly, from August 20 to September 18) is the last summer month and I don't know any relevant feast or celebration held during the same. Regarding celebrations it looks like a rather calm and quiet month, perhaps as a subconscious waiting/warning of autumn.
Anyway, the closest thing I've found in the Arcanarchiva™ is romallàs "flashing, lighting-like" (from romall "flash, lightning"), a popular name for this month; another one being tremareu "thundering" (from trem "thunder"). The reason is obvious: that is a stormy month, with the first rains after summer, as well as hailstorms and thunder storms.
According to popular belief such phenomena are caused by anyets, the atmospheric genies that throw lightnings and hailstones. Folktales tell how these anyets throw ice mountains among themselves, and when the mountains of ice clash they're broken into thousands of small pieces of ice, thus producing hailstorms and associated phenomena.
There are no special celebrations or pilgrimages to apaise the anyets, but people believe that laurel trees prevent lightnings to fall nearby, and this is why this tree is so common in rural areas and urban parks. It's also common practice to hang laurel leaves on the outer side of doors.
The rite of cutting the sky with a knife to terminate a growing storm is still common in many countries and shires.