This is the Time of Horses (or Horse-time) in the fictional 6th century world of y Ddraig.
So named because in the northern hemisphere oestrus (ovulation) is triggered in horses by the longer hours of daylight between June and August. The mares who became pregnant last Horse-time have carried their foals through winter and an eleven month gestation and they have begun giving birth so that, by the time of Horses, the woods are filled with foals leaping about. This moon horses are traded and celebrated and stallions start looking with interest at any mare without a foal at her teat (and some with!).
This is also the time of the summer solstice and so yet another week-long feast is conducted in which all but the most necessary work ceases to mark the height of summer and acknowledge/respect the waning of the year and the approach of winter.