But you can still prepare (as long as the servers come back up) and things to know:
Put games you are interested in in your Wishlist. This way you will be notified via email when they go on sale. If you don’t have email notifications turned on you can also just browser your wishlist page and it will show current prices of all games, including sales.
Do not buy a game that is barely on sale (<25%). Most likely the game will go on a steeper one day-only sale during the Summer Sale. These are “daily deals” (last for 24 hours) or “flash sales” (last for 8 hours).
Miss a daily-deal or flash sale? Most likely the game will be at a steep discount at the end of the sale as well.
The above is based off of previous Steam sales, which has been consistent.