Whip it. Put it back in the fridge. Make sure you use salted.
Why salted? (Speaking as someone who hates pre-salted butter.) I mean, must it be salted for your method to work?
For that matter, why keep butter in the fridge? If you use it fairly regularly, just keep a stick out in a covered butter dish.
better yet, use a French butter dish. 
Well, not, it doesn’t have to be refrigerated. But the question was posed as how to make it spreadable right from the fridge. You don’t need whipped butter if it’s room temperature.
Whipping introduces air, which can oxydize fats (“rancidity”). Salted butter inhibits this. If you’re going to use it quickly, then unsalted butter ought to work. But then again, if you’re going to use it right away, you don’t need to bother with refrigeration.
Oh, I beg to differ. There are LOTS of dishes, sauces and baked goods that you just can’t do with unsaturated oils. Pie crusts, biscuits, cookies, etc… all require some sort of solid fat, whether it be lard, butter, margarine, coconut oil, crisco, or something else.