Nazaré in summer (July to September) is a busy beach resort. The town beaches — Praia da Nazaré and Praia do Salgado — are wide and well-equipped, and the town fills with Portuguese families and European visitors. It is lively and cheerful, if crowded.
The big-wave season is October to March. The Praia do Norte giant waves require a significant Atlantic swell, which typically arrives from November onwards. December and January see the most consistent conditions. If you are coming specifically to see or surf the big waves, check surf forecast sites (Windguru, Surfline) a week or two ahead. The famous WSL Nazaré Tow Surfing Challenge typically runs between October and February, on a waiting period — check the World Surf League calendar for activation dates.
Spring (April to June) is a good compromise: warm enough to walk and explore, calm enough to have the town largely to yourself.