The Netherlands has a maritime climate — mild, damp, and changeable. Don't come expecting sunshine. What the Dutch describe as "summer" can involve grey skies and showers on any given day, and what they call "winter" is more grey and drizzly than properly cold. Snow falls occasionally but rarely settles. The one constant is wind.
Spring (March–May) is the most rewarding time to visit if you're interested in the famous flower landscapes. Tulips bloom from mid-April through early May, and the Keukenhof gardens near Lisse are open for eight weeks in this period. March is the driest month of the year, which makes it an underrated time to go — you get reasonable weather without the summer crowds. Easter weekend is busy.
Summer (June–August) is peak season. The days are long, the terraces are packed, and the city canals are full of boats. Amsterdam can feel overwhelmed by tourists in July and August. The weather is pleasant rather than hot — temperatures typically reach 20–25°C. This is festival season: King's Day is 27 April (technically just before summer), Lowlands music festival is in August, and Amsterdam Pride is held in late July or early August.
Autumn (September–October) is an excellent time to visit. Crowds thin out, prices drop, and the light on the flat landscapes can be genuinely beautiful. Expect rain, but also sunny days.
Winter (November–February) is the quietest and least expensive season. The cities have good Christmas markets, and Amsterdam takes on a different mood — quieter, more local. If canals freeze (rare), skating is a Dutch obsession. But it's cold, often wet, and the days are short.
For most travellers, late April to early May or September is the sweet spot.