I'm not at college yet, but I can tell you something about exams because my older brother is at second year.
They have partial tests throughout the year and final exams in July and December, which they have to pass in order to be able to do the next course. I don't think there are credits for doing extra tasks, but what I do know is that they usually have to present a paper and then a board made up of professors will ask them questions about it.
As regards summer holidays, they start at the end of November and finish on the first days of March. However, undergraduate students have to sit for exams in November and December, and may have to study during the holidays if they have to resit an exam. Their classes generally start in the middle of March.
Today, winter is starting here in Argentina, even though it has been very cold lately.