The Magearna event is currently ongoing. It can be obtained with a QR code. You can find the code on Serebii or the Pokemon website, make sure it's the code that matches your country though. You have to open up the QR scanner on the menu of your Sun or Moon game and scan the code. Then, go to the Hauoli Mall and get Magearna from the delivery man.
There is currently no Marshadow event yet.