Petersburg gets roughly 110 inches of rain per year, which is the price of living in a Sitka spruce rainforest. Rain gear is appropriate at any time of year.
Summer (June to August) is the best time for most outdoor activities: charter boat trips to LeConte Glacier and for whale watching, salmon and halibut fishing, hiking the Raven Trail, and catching the long northern daylight. Temperatures are mild — typically 15–18°C during the day. The Little Norway Festival on the weekend nearest May 17th (Norwegian Constitution Day) is the town's biggest annual celebration and worth timing a visit around.
Late May through June brings the peak of king salmon season; July and August the sockeye and coho runs. September is the end of the main season and can be excellent for fishing — fewer visitors and still-reasonable weather.
Winter is dark, quiet, and cold but not brutal by Alaska standards. The harbour is active year-round and Trumpeter Swans winter on the river flats. Petersburg receives some visitors who come specifically for the quiet and the dramatic winter light over the islands.