Samwell Tarly finally made it to Oldtown in the season six finale. But while Sam was checking in to become a maester, he said he had been sent there by Jon Snow, the current Lord Commander of the Night's Watch.

Little does he know that since Sam's been gone, Jon was murdered by his brothers of the Night's Watch, brought back to life by a red priestess, and then subsequently left the Night's Watch and his position as Lord Commander to wage an epic battle for Winterfell.

News really does travel slowly in Westeros.