Santiaguinos tend to abandon their city every summer during the school holidays that run from the end of December to early March. February is a particularly popular vacation time, when nearly everybody who's anybody is out of town. If you're not averse to the heat this can be a good time for walking around the city; otherwise spring and fall are better choices, as the weather is more comfortable. Spring and fall are also good times to drive through the Cajón del Maipo, when the scenery is at its peak.
Winters in the city aren't especially cold -- temperatures rarely dip below freezing -- but days are sometimes gray and gloomy. Ski season, depending on the resort, runs mid-June through mid-September. Good weather conditions, however, mean six-month-long seasons beginning in May and ending in October.