The best time to visit is mid May-late September, when day temperatures are generally in the 70s-80s F/23-32 C, with nights in the 50s-60s F/10-22 C. The winter months are cold -- in the 30s-40s F/0-10 C during the day and below freezing at night. It rains a lot year-round, so take a folding umbrella or a waterproof jacket. A sweater is necessary almost any time of year. If you're going strictly for indoor sightseeing, October can be very nice. If you want to avoid crowds, try mid April-mid June or mid September.