Original: In Poland, for example, it was a few days per year in the 13th century; one day per week in the 14th century; four days per week in the 17th century and six days per week in the 18th century.
Simplified: In the 14th century, serfs had to work one day per week. They had to work 4 days per week in the 17th century. They had to work 6 days per week in the 18th century.