Trekking is best from January to March. You can visit from spring to autumn, and April is often a glorious month, when the foliage turns scarlet and the days are clear. Southern Patagonia pretty much shuts down in winter, with the exception of the ski resort at Ushuaia. El Calafate and Ushuaia are way overbooked in January and the first half of February, so plan ahead.
NB, please do not rely solely on this information for your travel planning.
INTERNAL FLIGHTS: Try LATAM and Aerolineas Argentinas.
BY BUS: Long-distance buses take days to reach El Calafate from Buenos Aires, but if you're travelling from Argentina’s Lake District, consider the 14-hour bus ride down the iconic Ruta 40. El Calafate can also be reached by bus from Torres del Paine in Chile. Buses go on from there to Ushuaia.
For information on international travel to Buenos Aires, see our guide on getting to Argentina.