Have never seen Threads, and continue to have mixed feelings about whether I want to.   Possibly because I have seen Testament, a feature that came out the year before Threads and covers pretty much the same ground.  Testament seems mostly forgotten now, though Jane Alexander was nominated for an Oscar for it. 

It's not as sensational as I gather Threads is - it's a very small movie about a small town outside San Francisco after a nuclear war. Nothing spectacular happens, it's just two hours of watching things get worse and characters dying of fallout poisoning, with Jane Alexander trying to keep it together as the mom of a small family after dad never came back from work that day.     Saw it once in 1983 and some scenes are still burned into my memory. Although it's a great, well-made movie I've never felt an urge to watch it again.

So I wouldn't call this a recommendation per se, but hey, if you saw Threads and thought "gosh I hope there are more movies like this", then Testament is there for ya.

Of course there's also On The Beach, the original "well, the world is over so NOW what?" fallout movie from 1959, in which the residents of Melbourne wait to die from the aftereffects of a Northern Hemisphere nuclear war they weren't even involved in. 

So that'd be one hell of a triple feature, if you want to really cheer yourself up.