I'm just wondering out of curiosity. I'm still new to The Sims 4 and I got the base game in May, and I some packs when they were on sale in May and June. I really want to know when these sales usually...
Expansions packs go on 50% off sales several times a year.
Game Packs go on 25% off regularly during the year, but about once or twice a year they go to 50% off. So you have to patient if you want to save more money on those.
Stuff packs rarely, sometimes only once a year, go on sale for a few dollars off. This year was an exception.
You can also check other stores like Target.com for deals. I have purchased packs on decent sale (including stuff packs) on Best Buy, Walmart and Target.