Juneau gets a lot of rain — around 60 inches per year in the downtown area, and considerably more on the slopes above. The weather is famously unpredictable: warm and sunny days in May, cold and wet ones in July, and rarely anything that qualifies as genuine summer heat. Pack layers regardless of when you visit.
May and June are the best months. Daylight is near its maximum (over 18 hours at the solstice), the wildflowers are out, and the tourist crowds haven't fully peaked. The weather is still variable but tends to include more clear days than midsummer. The Mendenhall Glacier visitor centre is open, whale watching is excellent, and the floatplane operators are fully running.
July and August are high season — cruise ships arrive daily, and the downtown waterfront can feel crowded from late morning until the ships depart in the afternoon. Book accommodation and popular tours well in advance. Despite the crowds, this is peak wildlife-viewing time for humpback whales, bears, and spawning salmon.
September brings cooler weather, falling leaves, and noticeably fewer visitors. The fishing is good and the autumn colours in the forest are worth seeing.
Winter (October to April) is quiet, cold, and wet. Several sights close or reduce hours. What you gain is a genuine off-season Juneau — the bars fill with locals, the northern lights are occasionally visible on clear nights, and the surrounding mountains hold good skiing conditions at Eaglecrest Ski Area on Douglas Island.