Farfetch has sales basically all year round, often with some hidden gems. Yoox is also good… I’d also look for secondhand if you don’t already, I like Grailed and The RealReal especially for higher end items. Some of my friends talk about yjp as well.
matchesfashion, end, and luisaviaroma usually have something on sale (though only luisa carries D^2 ). their big sales are usually around after Christmas and the 4th of July. mrporter too, but they only have sales twice a year.