We were there from the 20th to 29th of Jan and I didn't wear anything but shorts and t-shirts. The mornings were a bit cooler, but the sun was hot in the afternoons and shorts were the best bet. I was wearing the shorts and shirts and I saw CM's wearing, toques, mitts and parkas one am.
Another thing about that time of month, there were almost no lines. 90% of the time it was less than 10 minutes, and a couple times we waited about 20-30 for the big attractions. I only used a fast pass once in 4 days.