The summer months make up chief of the tourist season. Tourists seeking to acquaint themselves with the Leaning tower of Pisa and the art and architecture usually visit the city during this time. Summer skies are blue and the weather sunny, making exploring the city on foot a pleasurable activity.
The winter months witness fewer tourists than summer. The temperatures are not severe and the sun comes out often. You should make it a point to carry an umbrella, though, for sudden rains during these months are not uncommon. Watching Pisa light up for Christmas is a delight.
The weather is pleasant and the prices are moderate. It is less crowded and you can explore Pisa in peace during these months.