When someone says something upsetting you'll say sad days. Things that make you sad -sad days.
I can't come over - aww sad days
I miss you. I'm having sad days here!
When one has run out of cannabis.
Person 1: "Dude, i just smoked my last bowl, and i'm outta money for at least a week :("
Person 2: "sad days, bro. sad days"