In the north-western part of Bosnia, a custom existed until the middle of the twentieth century called Uvjet (condition) according to which every Thursday from spring to August, when the wheat harvest is finished, was considered the largest of all holidays. No farming activities were undertaken on Thursday for fear of breaking this taboo one would cause bad luck and infertility. Allegedly, the one who was brazen enough to break this rule and would work in his field on Thursday, thunder and bad weather would destroy all of his wheat yields as well as other grains. But, as soon as the harvest period would end that taboo lost its importance and Thursday was regarded as just another day.
It remained written that certain Imam's strongly objected this pagan practice, there was even physical violence with the villagers, since they didn't want to quit their ancient practice under any circumstances. In the end they decided to go and see the main Imam in Travnik so he too can officially give permission for them to observe this custom, in which they succeeded.
That it is not an isolated case is clear from the data that in Velika Kladuša we find traces of the so called cult of the Thursday which was clearly dedicated, since the Illyrian-Roman period, to the god Jupiter god's of storms, thunder and lightning, which corresponds to the belief from Velika Kladuša that Thursday was called "white Thursday" from spring and on that day it was forbidden to wash underwear, especially white ones, until the grass mowing begins, and all out of fear that bad weather will destroy wheat and other agricultural crops. Behind the epithet white Thursday lies smybology which denotes holly or holiness, so that white Thursday is the same as "holly Thursday".
Since the beginning of May in Celtic but also Bosnian tradition signified Beltaine, beginning of summer, i.e. the light part of the year, which was renamed, with the advent of Christianity, into the holiday of St. George (Hidirlez), it is easy to reach a conclusion about the ancient dedication of Thursday to a celestial deity. With the Roman occupation of the Illyrian kingdom (Illyricum) and mixing of the roman religion with the Illyrian there were some changes when it comes to positioning of god's in the Illyrian pantheon and Jupiter replaced Vidasus. For the Illyrians, Vidasus, with his companion goddess Tana, was the main deity which ruled over fertility and nature. All this leaved enough space to conclude that the Illyrians considered Thursday as the day of Vidasus, their largest deity. That's why the custom of Uvjet is one of the best kept segments of the former widespread Illyrian cult of fertility among the Bosnian people.