January and July are when the Argentines flock to Bariloche primarily because those months coincide with the major school holidays. In January, particularly, the portenos (port people) love to leave the capital Buenos Aires and head for the interior and July is when the ski season is already in full swing. If you want to avoid the crowds then June or September might be a better time to ski at Cerro Catedral. Trekking around the Parque Nacional Nahuel Huapi is best between December and April, although some of the higher treks might still have snow.