Blizzard has already confirmed the return of the anniversary event last month but just in case you forgot about it, The Darkening of Tristram returns.
If you’re looking for some nostalgia, or can’t be bothered to install Diablo 1, then this is the closest you can get to the classic original in Diablo 3. The event runs from 1 January until 30 January so there’s a month to roam around inside the Cathedral. Alternatively just install Diablo 1.
If you want to know more about the event, head to the Diablowiki The Darkening of Tristram page where rewards and other useful information is available.