If you are planning a visit to Seville, it is best to go there during the Spring months, from March until May, and Fall, from September to November. During these periods the weather is mild and pleasant, and you can avoid the heat and the crowds as well.
During Spring time, the Feria de Abril festival takes place, which celebrates the Andalusian culture and is prominent for its colorful parades. The city is also decorated with beautiful flowers, there are flamenco performances and lots of music and dances. Thus, you might consider coming to the city in April, when the festival is set to be.
Then, during the Fall, there’s the Bienal de Flamenco, which is a showcase festival of Flamenco. You can get to see some of the best Flamenco performers and experience the rich cultural heritage of Seville.
The summertime is not such a good time to visit the city, as it is quite hot and humid. However, it does not mean that you should not go if you have plans already. On the contrary, the winter period can be quite cold, so it might be quite unpleasant to visit most of the sights.
Overall, visiting Seville, really depends on what you are looking for and your preferences. So, I hope this summary helps!