May to September is when Leeuwarden is at its best — the canal terraces are open, the Prinsentuin is in flower, and boat tours run regularly. July and August can be busy with Dutch tourists but never crowded by international standards.
If you can time it: late summer brings the Elfstedentocht memorial events (in years cold enough, the legendary 200km Eleven Cities skating tour itself), and the Bloemenmarkt in September fills the Wilhelminaplein. In winter the canals occasionally freeze, and Leeuwarden becomes a base for skating excursions out into the Frisian countryside. Daylight is short and many sights keep reduced hours November through February.