As Barcelona enjoys the typical Mediterranean balmy weather, there is no one season that you should beware of. However, spring and autumn (fall) are ideal times to visit and most especially the months of May and June with temperatures reaching in the 70sF (21°C) as well as September and October when the temperatures average lies between the mid 60sF to mid 70sF (18° to 22°C).
In winter you are almost guaranteed of sunny days, but they will be influenced by the mountains and so warm clothing will be a must. You might experience some snow but it never lasts more than a day or two. Temperatures at this time of the year rarely fall much below 50°F (10°C).
The time for rain is in April, but there are some spectacular storms in July and August when the daytime temperatures reach around 55°F (13°C), so although it is not particularly hot the resulting humidity can be rather uncomfortable especially as the nights don’t bring much relief.
The sea temperatures in June through to October make swimming a real pleasure. So planning a day on the beach would be a pleasurable addition to your itinerary.
August is a time that you should be wary of, as it is Europe’s main holiday season and this means that there is a positive migration to Spain from France, Germany and the Netherlands and unless you book well in advance you will have trouble finding low-cost accommodation. It is also the time when most Barcelonans take their holidays and head off to the cooler regions which means for you the visitor, that many of the usual haunts will be closed.
As Barcelona is a major centre for international expositions and conferences throughout the year you are well advised to book your accommodation in advance whether it is a mid, or high range.
Christmas is a time when most stay home so if this is the time of year you have chosen to travel you will be pretty much without the company of fellow travellers.