Used to be, when bike racing began and ended in Europe, the season started with Milan-San Remo or Paris-Nice, or some would say Het Volk. But now, the big teams show up for the Tour Down Under, Tour of Quatar, Tour de Langkowi, and now the Tour of California. I feel like I'm forgetting some more. Majorca?
The season used to be March to October, with winter training camp in January and February. Pro racers rode the entire season. Now, there are races (and money and points) year-round, and riders focus and train for the specific parts of the season that suit them the best. For example, the CSC team that races the Tour of Flanders will be different from the one that races the Vuelta, etc. Only time there isn't much road racing is December now.