Carlisle's weather is honest northern English weather — mild summers, wet winters, and the possibility of a cold wind off the Pennines at any time of year. Summer (June to August) is the most comfortable for walking the walls and visiting the castle; days are long and the light on the red sandstone buildings is particularly good in the evening.
The Carlisle Great Fair, held in August, is a centuries-old tradition. The Settle-Carlisle Railway runs steam excursions at various points through the year, which are worth timing a visit around if you are a railway enthusiast. The city is also a useful base for the Lake District or Hadrian's Wall country, and both are best in spring or early autumn when the crowds thin out.
In winter the city is quiet and a little grey, but the cathedral is warm and the pubs are good.