Pokhara has three seasons that matter to travellers. October–November is the post-monsoon peak: clear air, clean mountain views, perfect trekking weather, and the most crowded Lakeside of the year. Book accommodation ahead in October. March–April is the second-best window — rhododendrons flowering in the hills, warming temperatures, generally clear views. June–September is the monsoon: thick cloud, leeches in the hills, occasional spectacular thunderstorms, and the mountains hidden for days at a time.
Winter (December–February) is also good for visiting — cold mornings, warm afternoons, very clear views — but the high passes on the long treks are closed by snow. Whatever the season, expect mountain views to be best between sunrise and about 10am, after which haze and cloud usually build.