Fashion in the stores is about 2-3 months ahead of the actual season, not always "two seasons ahead."
For example, fall merchandise (boots, long-sleeved shirts) will start appearing in stores in July or August and heavily promoted in September, but true winter clothing (down coats, snow boots) won't appear until closer to September.
This varies based on geography; the farther north you go, the earlier cold weather clothes will appear--because cold weather itself starts sooner.
In general, as Trace Evans notes, this lets people plan and buy ahead.
There's also a psychological component. By the time it's February, the last thing a shopper wants to see is another stupid sweater. We're ready to bring on the bright colors and new spring styles.
This advance merchandising is why Fashion Week is always one fashion season (two calendar seasons) ahead: Fall 2012 collections were just shown in New York. The Spring 2013 lines will show in September.