Summer (usually late June to early July) and Winter (late December to early January) sales are the major sales. They are the big ones, dwarfing even this Golden Week sale.

Almost all steam games would be discounted during then. Almost (exception are a few holdouts like Factorio who will never go on sale according to the developer, and new titles of course)

There will also be weekend sales, mid week madness sales, and a seperate weekly sales. Although these are usually limited to smaller indie titles, you will occasionally find a good game here.

Then there are also sales propped by the publishers/developers, which you can expect when they are releasing a major title or celebrating an anniversarry. For example, Game X will be one year old on a certain date, expect it to go on a discount by then. Or for example Star Wars games are on sale now in commemoration of May 4th (which is star wars day)

Then you also have seasonal sales, like Chinese new year or valentines or Halloween sale, although these are limited titles matching their theme. For example, only spooky/ghost tiles will be on sale during Halloween sale. Or this Golden Week sale, it's in accordance to the Japanese Golden Week that's why you mostly see Japanese/Anime titles in there.