Today, a Raven will spawn in one of 9 locations. Note that the location will be the same for everyone on that specific world; different worlds may spawn it in different locations.
Cadarn - on the ledge South of the spinning wheel building, just across the Max Guild gate
Cadarn - on the ledge by the window outside Haf's Battlestaves, near the range
Crwys - just behind Coeden's building, near the Magic tree
Crwys - behind the building East of the spirit tree
Iorwerth - on the Southern ledge of the building East of the Iorwerth church
Iorwerth - on the ledge between the church and the building to the East; best seen from the South East
Iorwerth - North of the building West of the Iorwerth church
Iorwerth - South of the building West of the Rush of Blood entrance
Trahaearn - between the forge building and the armor display building, South of the Seren stones
The Raven will remain for the full day. After clicking the bird, you will receive a chatbox message that reads: "You have found X/13 ravens. I wonder what happens when you find them all?"
Upon finding your 13th Raven, you will unlock the "[Name] the Ravensworn" title and be able to breed animals on your Player-Owned Farm with the "Ravensworn" trait.
Note that the calendar automatically converts it to your timezone. For example, if the Raven spawns at reset on the 16th, which is 7PM on the 15th for me, the calendar will list the 15th when I look at it.