The main pilgrimage dates are 13 May and 13 October — the anniversary of the first and last apparitions. Crowds on these days can reach several hundred thousand, and accommodation in Fátima and the surrounding region fills months in advance. If you want the full pilgrimage experience, plan well ahead; if you want a quieter visit, avoid these dates.
The night before each pilgrimage date is particularly atmospheric: a candlelight procession fills the esplanade, with the crowd carrying candles and singing together. The experience is difficult to find elsewhere and worth planning around, even if you're not religious.
Outside the main dates, smaller pilgrimages on the 13th of each month still bring significant crowds. Fátima is busy year-round, but weekdays in the winter months offer the most peaceful visits. Weather in central Portugal is mild: warm and dry from May through September, wetter in winter.