Cheating, accidental pregnancy, and suicide don’t really sound like too much fun. In fact, they sound like major downers (maybe that’s why the band uses a period instead of an exclamation point). But that’s exactly what the band fun. sings about in their hits. And on Tuesday, they take the stage to perform “Some Nights,” “We are Young,” and “Carry On” — basically the soundtrack to 2012 and 2013. Rounding out a summer full of big names, Family Circle Stadium wraps up its summer concerts before some more top-billed acts, like Widespread Panic, hit the stage in the fall. Pop duo Tegan and Sara share the bill for what should be a — yeah, we’re going to do it — fun night.