February is the most famous time to visit: Cádiz Carnival is the liveliest in mainland Spain and one of the great street festivals of Europe. For ten days the entire city dresses up and sings — not just in costume, but in elaborate satirical musical performances called chirigotas and comparsas, groups that spend months writing and rehearsing original songs mocking politicians, celebrities, and current events. The humour is sharp, local and mostly incomprehensible to non-Spanish speakers, but the atmosphere is joyful and almost entirely good-natured.
April and May are excellent: the weather is warm, the streets are quiet by Andalusian standards, and the beaches are still uncrowded. The festivals of Semana Santa (Holy Week) are observed here with the same intensity as in Seville.
July and August are hot (regularly 35°C) and busy. The beach fills with Spanish families on summer holiday and prices rise accordingly. The old city stays pleasant — sea breezes make it more bearable than inland Andalusia — but book accommodation well in advance.
The sea is warm enough for swimming from June through October. The Atlantic here is consistently cooler than the Mediterranean, but pleasant from mid-June.