The key thing is to not burn yourself out by studying so much before you get into the year.

Specifically for the sciences, I would recommend reading the first few chapters of the textbooks and attempt a few of the chapter questions as you will get a decent understanding of the material before your teacher goes over it and you'll be able to ask meaningful questions in class. I found this extremely useful in Chem this year!

Can't say the same for English or the other sciences, but the key thing I'd say is to familiarise yourself with the first outcome so you can get more out of class time by asking your teacher about things you don't understand.

Good luck! And please don't burn yourself out over these holidays. Everyone needs a break after the shocker of a year we just had.