Seville in April is orange blossom and processions. Seville in August is a reason to stay indoors until 8pm. Choose on purpose.
March–May
The best walking weather. Semana Santa fills the streets; Feria de Abril follows. Rooms and flights spike. Book both if those weeks are the trip.
June–August
Temperatures of 40°C are common. Monument mornings still work. Afternoon sightseeing does not. Hotel rates can drop in August when locals leave — heat is the trade-off.
September–November
The second-best window. Crowds thinner than Easter. Still warm enough for evenings outdoors.