Częstochowa can be visited year-round, but the character of the visit changes dramatically depending on when you come. The great pilgrimage seasons are August 15th (Feast of the Assumption) and August 26th (Feast of Our Lady of Częstochowa), when hundreds of thousands of pilgrims arrive. The walking pilgrimages from Warsaw, Kraków, and other Polish cities reach the city on these dates, and the monastery and the avenue are packed for several days around each feast.
If you want to experience the pilgrimage energy, come for these dates — it is one of the great folk-religious spectacles in Europe. If you want to see Jasna Góra itself without being swept along by crowds, come in spring or autumn. Winter visits are quiet and austere, with the monastery grounds largely empty and the city businesslike.