I've been randomly checking airfares for a December trip from my home airport (GSP, Greenville-Spartanburg, SC) to Stuttgart, Germany on my Delta app. The price has been running over $1300. Last night, it was $793. Bingo. Buy time. I decided to check to see how extensive the airfare sale is on the US side, so I plugged in Columbia (SC) and Charlotte to Stuttgart -- both were around $790. Atlanta, Birmingham, Raleigh-Durham, Lexington (KY), Charleston (SC), Jacksonville (FL), Orlando were all greater than $1300.
I decided to see how expansive the sale on the Europe side is. Every destination I plugged in from London to Ljubljana (except Gdansk, Poland) was priced in the $700 to low 800 range from Sept through mid March.
I decided to check to see if maybe Delta was matching someone else's fare sale on certain routes, but Expedia and Kayak show the cheap fares only on Delta and its partners (KLM, Air France).
It seems kind of bizarre that one narrow region (not even one state, as no cheap fares out of Charleston) on one airline has a fairly massive fare sale, while most places do not. It seems really... well... random! My experience is that the deep discounts -- at least on Delta -- tend to pop up for a day or two at a time 6-8 months before the trip on a Saturday morning or on a Tuesday. I'm headed to Switzerland for $680 round trip on Sept 30 due to a similar find on January 21.