Complex sentence: Open from late March/early April until at least mid-November depending on weather, during the annual week-long Sturgis Motorcycle Rally every first full week of August, the saloon can average 20,000 guests a night.
Split into simpler sentences: Open from late March/early April until mid-November (weather permitting), and is busiest during the annual week-long Sturgis Motorcycle Rally. Each year, the first full week of August marks the beginning of the Sturgis Motorcycle Rally, during which time, the Saloon can average 20,000 guests a night.