Safe to put nail into drywall when stud finder finds "live voltage"?
Nail isn't exactly safe if you are directly nailing.   Chances of something happening are small (and really much greater chance to damage line than "shock" yourself) but it is there.   Also I doubt a nail would hang the calendar without eventually sagging.   Calendars are touched and moved and not ideal for just a nail - unless you can get that nail in framing.   

Monkey hooks should be about as safe as it gets.   Don't hit them or press them in.   Screw them in slowly (I know there isn't threads) and once you pierce the drywall loosely move the main shaft up.  If you aren't applying pressure to it once it is in even if it hit a line it should not nick or pierce it.   I would go this route if you have to have the calendar in this exact location.   Monkey hooks are very strong and not invasive to your walls.   

If you want to be extra safe with Monkey hooks you can bend the tip so it isn't sharp on the end once you get through the wall.