In the 16th and 17th century Jarosław experienced its heyday. This was due to the development of trade, and especially of the famous Jarosław fairs. They were regarded as one of the largest in Europe
For the development of trade a favourable location of the town at the intersection of trade routes was of vital significance. The largest Jarosław fair was held in August - a four-week fair. The fairs attracted crowds of merchants not only from Europe but also from the Middle East. They traded in, among other things, salt, wheat, fabrics, leather, metal products, jewellery, spices, etc.
What is more, huge amounts of cattle were rounded up. It happened that in one year the numbers amounted to 40,000 oxen and 20,000 horses. An important role was played especially by trade in oxen thanks to which Jarosław gained considerable fame.
Jarosław fairs had a significant influence on the town construction industry. Old tenement buildings of Jarosław type are characterized by the shed (a kind of internal courtyard), lit by windows or skylights derived above the roof of their vertical extension. Cellars, the so-called merchant warehouses were also useful for commercial purposes. They were hollowed in loess. They were used for storing goods imported for the Jarosław fairs.
Of significant importance was the setting of the city on the navigable, port San river. Jaroslaw traded with Gdańsk; the most important commodity was grain. Duchess Anna Ostrogska navigated to Gdańsk about 3 tons of grain annually. From Gdańsk to Jarosław mainly food products were imported, which included: herring, salted fish, wine, fish oil, spices.