At the Weyrs (well, DCW in this specific case) do restdays automatically get observed the day after Threadfall if Thread Falls on the restday?
It is up to the Weyrleader to designate the restday. I would ask the Weyrleader of the particular Weyr directly what their policy would be. Sometimes there are special circumstances which would make them decide to change the day. So if you have a particular plot in mind, I'd shoot them off an e-mail and check with them to be sure.