El Rocío splits into two entirely different experiences depending on when you visit.
For the Romería, you need to be here at Pentecost — the seventh weekend after Easter, usually in May or June. This is when the pilgrimage arrives and the village transforms from ghost town into one of the most intense folk-religious spectacles in Europe. Book accommodation (in Almonte or Mazagón, as El Rocío itself has almost no rooms) months in advance. Expect crowds, noise, and mud if it has rained.
For birdwatching and peace, come between October and March. The lagoon is full of wintering wildfowl, the weather is mild, and the village is near-empty. Spring (February–April) sees the marshes at their greenest and the first returning migrants. Summer is hot, dusty, and quiet in a different way — the lagoon may shrink, but the surrounding landscape has its own scorched Andalusian character.