Well apparently there is a "May Sale" right now…
In the past there were 2 big annual sales, one around the winter holidays and one in early-summer – that's just anecdotal. There may have been more.
Before that there was a 'daily deal' but – again, anecdotal – I think Unity got rid of it when it caused problems with asset creators. I'd like to think Unity also didn't want to become an impulse-buy shopping network….
Also, some asset creators routinely rotate their items with temporary drops in their product price (doesn't show up as a 'sale' item).
Anecdotal shopping strategy: very popular items will be discounted potentially a few times each year (it's part of the reason they are so 'popular'). Less popular items may never be discounted.