Besides the different activities you can do in Pamplona, there is 1 mayor festival that you just have to participate at: the festival of San Fermín!
The festival of San Fermín is an annual historic event, from 12:00, 6 July, till midnight of the 14th of July. Traditionally, the big party ends with the singing of the 'Pobre de Mí'.
Of course, the most famous part of the whole festival, is the running of the bulls (encierro). This starts at 8:00 am from the 7th of July till the 14th of July.
The webpage of the official 'Spanish fiestas' also has a whole part dedicated to this specific event, with an extensive history and a lot of practical tips and tricks. Besides the encierro, there are still a lot of other folkloric events during the festival, which is held in honour of Saint Fermin (the saint of Navarra).
Of course, Pamplona is really proud on this big event that attracts every year a huge amount of tourists. 'Kingdom of Navarra', an official webpage of Pamplona, published the whole history around the San Fermín festival. So, if you are interested, be sure to go and take a look!