If I'm doing this correctly, your itinerary has you in the following ports on the following dates:
12/21 Santiago (Valparaiso)
12/23 Puerto Montt
12/24 Puerto Chacabuco
12/27 Punta Arenas
1/1 Puerto Madryn
1/4 Buenos Aires
According to The Weather Channel (weather.com), the forecast for Santiago on Sunday 12/21 will be sunny, high 85F/low 56F.
Puerto Montt on 12/23 has a 40% chance of showers, high 62F/low 50F.
As I can't get forecasts for more than 10 days out, the rest are average high and low temperatures for some of the rest of your ports:
Punta Arenas: avg high/low=56F/43F
Buenos Aires: 83F/68F
Others who have been there can advise on proper attire, etc. I've found that layers are often the way to go, especially as it cuts down on the packing. Expect it to be cool in the Chilean Fjords, around the Cape, and in the Falklands.