The question of when to go depends on what you want to do, really. Skiing is recommended from betweenChristmas and New Year′s Eve (this is also the time at which many Austrians join the tourists on the
slopes), as well as from late January to late March. This is the main season, when prices and ski traffic are high.
The low season runs in good years from as early as early November until Christmas, the early weeks of January and from late March until end of April. Especially during low season you can possibly encounter
snow-less mountains in warm years, so if you are a last minute person, it′s worth checking the snow situation before heading off.
The warm and pleasant summer months range from May to September with the summer peak in July and August. You can get warm and sunny days in April and October, as well as miserably cold and rainy ones during the summer, but there′s always an element of risk with the weather in Austria. My personal recommendation for sightseeing and outdoors things (hiking, national parks, sports) would be the months May and September - relatively few tourists, low airfares and other prices, and the weather is already or still pleasant.
Whatever you do, dress in layers. Goretex jackets with removable fleece are very popular among Austrians, and there′s a reason for that. Jackets of this kind will also prove to be useful when it comes to hiking: alpine terrain can be chilly even during the hottest summer days.
Recording the weather, by the way, is a business with tradition in Austria. Detailed meteorological surveys and measurements are made and archived since the 17th century. The summer of 2003 was the hottest ever measured - so far.