You’ll have some things going on in El Calafate in late May although walking over the Perito Moreno is uncertain due to snow/rain. You should also take into account that days will be short – sunrise is around 9am. El Chaltén is more difficult as it’s all about trekking and many paths close in late April due to snow and heavy rain. You may be lucky, though, and get beautiful sunshine, weather is unpredictable in Patagonia. Except for the cold – it will be very cold.