At the Peace Center in Greenville, SC.* Starts at 9pm EST on CBS.
Of course the candidates will have to mention Justice Scalia (who died today) and talk about who they would appoint to the SCOTUS. (Cruz already has declared he will oppose whomever Obama appoints.)
*The South Carolina primary is 02/20/16 for Dems, 02/23/16 for Pubs.