It's fairly difficult to know exactly when things will go on sale or what exact items will be on sale in the crown store. I'd say the most important thing to know is which items you can earn or buy in game to save your crowns for things that are store exclusive.

For the most part, any of the cosmetics (including costumes, mounts, pets, hairstyles, etc) in the Crown Store are exclusive to the store. The crafting motifs and storage containers are available in-game, as are the skill lines. Some of the service tokens like race/name/alliance tokens are store-only, but then there are things like Riding lessons/XP scrolls/respec scrolls that are available in-game with cooldowns. Crown Crate goods are also store exclusive, save for the simple stuff like potions (which should generally be sold for gems anyway) or crafting motifs that can be obtained in-game. As for houses, most are available in-game for gold, but most in the 'notable' category are crown exclusive.

We get Crown store showcases every month to show what's going to be in the store that month. It usually drops the first workday of the month or a few days earlier, so October's should be coming tomorrow.

Crown crates are on a three-month cycle. Sometimes the 15-crate pack will go on sale for ESO+ members in the last week of availability, but that's not consistent and sometimes it doesn't.

There are a lot of store-exclusive things that take forever to come back though - they really subscribe to FOMO marketing here. There's also no reliable guarantee that anything will come back in a reasonable time either. About the only things you can rely on are the Daedric weapon styles tend to drop the few days over the respective Daedra's summoning day, and the Iceheart weapon pack goes up areound the winter holidays. Anything else... well, you can post in the wish list and pray.

The Crowns themselves have some sales that usually fall during the winter holidays, during the game anniversary in April, and during Quakecon in August. Of course none of those are guaranteed, but those have been pretty consistent so far.

I think they usually have a Black Friday sale sometime in November, other than that I'm not sure. But there's often a few items discounted in the crown store each month and I don't think there's any fixed schedule for that.