The Lake District weather is unpredictable year-round, but the best conditions for the big fell walks are late spring (May-June) and early autumn (September-October). May and June have long days, relatively dry weather, and the fells are free from the summer school holiday crowds. September and October bring autumn colour to the lower woodland and the best light for photography.
July and August are the busiest months. Glenridding's car park and the Helvellyn route can be uncomfortably crowded on fine summer weekends. If you are planning the Striding Edge ascent, a midweek day in May or September is significantly more pleasant than a summer Saturday.
Winter walking is possible and rewarding but Striding Edge in icy or snowy conditions requires crampons and ice axe skills. The lake is beautiful in winter and the Ullswater Steamers run a reduced service year-round.
The daffodils along the Ullswater shoreline near Gowbarrow are worth the trip in late March and early April.