It doesnt matter when you are going to visit Barcelona. It is beautiful at any season and the weather is almost always fine there. During holidays, especially Christmas, there are lots of tourists in Barcelona and its better to buy your Barcelona city tour beforehand.
Most Barcelona tours are 8/7 (That means they last 8 days and 7 nights.) but you can also buy a 11/10, 12/11 or 15/14 tour. If you plan a winter trip to Barcelona, you will probably see one of its winter carnivals. They are really fascinating! You will see a lot of fancy dressed people and hear a lot of music in Barcelona streets during these days. Aura of joy and fun will follow you everywhere.
If you go to Barcelona in May, visit its spring festival where they decorate so called May tree, dance sardana, and do circle dances. If you like folk dancing, remember in any season you can see it on Pla de la Seu where Catalans dance sardana every weekend. It is really fantastic to see that! By the way, you can also dance with them.