Lucerne is a year-round destination but the season matters. Summer — June through August — brings the Lucerne Festival, one of Europe's major classical music events, as well as warm lake swimming and good visibility for the mountain views. The Kapellbrücke and old town are busiest in July and August; if you prefer fewer crowds, May and June are excellent months with long days and mild temperatures.
Autumn is underrated. October brings clear skies, lower prices, and the city largely to residents. The lake and mountain views can be at their most dramatic when the first snow caps the peaks.
Winter in Lucerne is mild by Swiss standards; the lake moderates temperatures and the city stays largely ice-free. Christmas sees decorations and a market, though the main draw for winter visitors is the proximity to ski resorts in the surrounding Alps, all reachable by train or boat.