Table:
text: 1997 IMSA GT Championship season
Schedule
Schedule
Race Length_ Class_ Circuit Date_
California Grand Prix 1 Hour 45 Minutes GTS Sears Point Raceway July 13
California Grand Prix 2 Hours WSC Sears Point Raceway July 13
Mosport Festival 1 Hour 45 Minutes GTS Mosport International Raceway August 31
Mosport Festival 2 Hours WSC Mosport International Raceway August 31
Sportscar Grand Prix 1 Hour 45 Minutes GTS Las Vegas Motor Speedway September 20
Sportscar Grand Prix 2 Hours WSC Las Vegas Motor Speedway September 20
Festival of Road Racing 2 Hours WSC Pikes Peak International Raceway September 28
Festival of Road Racing 1 Hour 45 Minutes GTS Pikes Peak International Raceway September 28
NAPA Sebring Octoberfest 2 Hours WSC Sebring International Raceway October 18

text: 1997 IMSA GT Championship season
Schedule
Schedule
Race Length_ Class_ Circuit Date_
NAPA Sebring Octoberfest 1 Hour 45 Minutes GTS Sebring International Raceway October 18
VISA Sports Car Championship 1 Hour 45 Minutes GTS Laguna Seca Raceway October 26
VISA Sports Car Championship 1 Hour 45 Minutes WSC Laguna Seca Raceway October 26

text: 1997 IMSA GT Championship season
Schedule
Schedule
Race Length_ Class_ Circuit Date_
Rolex 24 at Daytona 24 Hours Both Daytona International Speedway February 1 February 2
Superflo 12 Hours of Sebring 12 Hours Both Sebring International Raceway March 15
NAPA Grand Prix of Atlanta 2 Hours GTS Road Atlanta April 19
NAPA Grand Prix of Atlanta 2 Hours WSC Road Atlanta April 20
Dodge Dealers Grand Prix 1 Hour 45 Minutes GTS Lime Rock Park May 26
Dodge Dealers Grand Prix 1 Hour 45 Minutes WSC Lime Rock Park May 26
First Union Six Hours at the Glen 6 Hours Both Watkins Glen International June 1

Question: On which date of the 1997 IMSA GT Championship season was a GTS class held at Mosport International Raceway?