I just thought I should say a few things, given both the HSFY 2019 and HSFY 2020 threads have had a few posts regarding studying HSFY before HSFY.
It is not necessary.
It is not necessary to prepare for the content of HSFY. You would be better off working out your learning style, and devising study strategies to maximise your learning.
In my view, the challenge of HSFY is not actually the content, but the fact that you're at university. You will likely be away from friends and family (and have to make new friends - not helped by the competitive setting), and you are in a university-type education situation. You generally don't have a "teacher" to give you individual attention or come around chasing you up if you haven't kept up. At university, you are responsible for your own learning; help is available but you have to ask for it - there isn't someone that's going to check up on you daily to make sure you've done your "homework".
Each "paper" you do (4 in the first semester, and 3-4 in the second) is a separate entity, with its own programme of lectures, labs/tutorials, assessments, etc. The programme will run at the same speed regardless of where you're at, whether you're keeping up, and whether you were drinking the night before and too hung over to concentrate on the lecture. Speaking of "drinking the night before", at university you are also in a university-type social situation. As part of being away from family, many of you will taste "freedom" for the first time - the ability to decide to go out with friends and not study tonight, and that freedom can be quite dangerous if you overindulge.
You may also end up with a fair amount of time between classes - you might have an 8am lecture, then an 11am lecture, a 1pm lecture, and a 5pm lecture (that's what I used to have in first semester all those years ago - 8am HUBS, 11am CELS, 1pm CHEM, 5pm PHSI), and what you do inbetween those lectures is entirely up to you. It's a lot easier to say "oh I'll have an extended coffee break and then watch some YouTube for a while before the next lecture" than to hunker down and go over the lecture you've just had and make notes for it - and over an entire academic year, all those extended coffee breaks might be costing you your place in medical school.
[Disclaimer: below HSFY commentary based on my experience of HSFY in 2011, things may not necessarily be the same now, although I'd imagine it to be reasonably similar now - perhaps with improvements.]
Classes wise, for each "paper" (subject) you have generally 4 weekly lectures (so 16 lectures a week between your 4 papers), and one lab session every two weeks (so roughly two labs a week between your 4 papers) if I'm remembering right. The lectures are 50-min lectures where the lecturer has a 45-50min presentation to give, and they aren't stopping for you if you couldn't keep up or got distracted. You do however get the lecture slides (the powerpoint presentation itself), and they record the lecture on audio and/or video for you to play back later.
The lab sessions are meant to be 3 hours long, generally with mandatory preparation beforehand. Once inside you could expect some further explanation/exploration of content covered in lectures, and some practical elements (chem labs feature experiments, for example) to help you learn/contextualize the same content.
[End of Cathay's "recounting HSFY 2011" ramble]
My recommendation to you, during the summer holidays, is to figure out how you learn best. Take some learning-styles quizzes, figure out a system for studying - is it extended written notes that work for you, or mind maps and similar diagrammatic stuff? Do you learn better by reading something on paper, or listening to someone tell you something? (I learn much better by listening than reading dense textbooks, so attending lectures was essential for me.) Or do you have to say things out loud over and over to remember them? Will you keep laptop/tablet-typed notes, or hand-written ones? Do you need to buy a laptop/tablet/etc for uni?
I guess what I'm saying is, you would be better served figuring out how you learn best, and working out how you'll go about "studying" (the most nebulous word in uni), than you would trying to learn the content. It also pays to set up a system for how you will spend your time - it pays to use time efficiently. I'll give an example of how I used to spend my day during HSFY below:
During HSFY I lived off-campus, so I used to force myself to be at uni from 8am to 6pm, and I used the time inbetween lectures to study - go over the lecture I've just come out of, print out the lecture slides, type up my own notes using the slides and my hand-written notes taken inside the lecture. Then if I get to the end of my attention span I would switch to something else like lab prep, or go over notes for a different subject. In this way, I made sure that I made notes and kept up with every lecture on the same day, never falling behind, for if you fall behind, it's a hell of a task catching up while keeping up with new content - I caught the flu once in second semester, and missed a day's lectures, and with 4 new lectures a day it took me most of a week to be back on top of things.
Admittedly I didn't actually study much in the evenings (that's when I spent time on MSO!), and only made a half-arsed effort in the weekends. But by making full use of what I used to call my "work day", I only really needed to work hard in evenings and weekends leading up to tests and exams; and on the flip side, because my "work days" were fully utilized and my evenings/weekends were cruisy, when it came time for tests and exams, I had big chunks of extra time I could devote to them - which I wouldn't have had if I wasted away the hour gap here and two-hour gap there during the day and worked hard in the evenings.
(Oh, how I wish I'd kept up the same study habits in med school. I still insist to this day that HSFY is the hardest I have ever worked.)