On a daily basis, all year round the euro to Peruvian Nuevo Sol prices change in foerign markets, depending on the strength of the Peruvian Nuevo Sol currency against the Euro.
This is why, there is no best moment to purchase soles. But you may find free websites like Cambiator that support you by comparing all the soles offered by currency suppliers, the airport and the banks. With these rates you will succeed in buying soles at the best price each time.