San Martín's has a fabulous location - the town is situated between dry steppe and the lush lakes district - which means the weather is generally sunny year-round.
The summer months, mid-December through January (Argentinian’s favorite month for travel) and February, are the busiest time of year for the region. Visit in the fall (from late April to May) if you want to see the leaves changing, or in the spring (November to early December) if you want to see wildflowers in bloom. 
San Martín offers many snow-related adventures in the wintertime, but any traveling you've planned may be hampered by snow storms and other inclement weather.