Seville experiences blistering, hot summers where the heat sometimes becomes unbearable. June to September are the hottest months of the year and there is no rainfall during this time. The winter months are mild but not unpleasantly cold. You will still have beautifully clear, sunny days in winter. The rainfall usually starts around October but rainy days don’t last long here.
The best time to visit Sevilla would have to be during the months March to May. This is when the weather is the most moderate and it is unlikely that you will experience any unpleasant conditions.