Per the official Spurs press release:
The San Antonio Spurs today announced that they will retire Tim Duncan’s No. 21 jersey on Sunday, Dec. 18 when the Silver and Black host the New Orleans Pelicans at 6 p.m. CST. A special postgame ceremony honoring Duncan will take place on the AT&T Center floor and will be broadcast live on FOX Sports Southwest (also streamed live on FOX Sports GO).
Duncan will become the eighth Spurs player in franchise history to have his number raised to the rafters, joining Bruce Bowen (12), Sean Elliott (32), George Gervin (44), Avery Johnson (6), Johnny Moore (00), David Robinson (50) and James Silas (13).