the Sat/Sun column are for Twin Sprint weekends. On a twin sprint weekend, every sprint race is run twice, once on sat, once on sunday. So if you check off Sat, you will race on Sat, if you check off Sun, you will race on Sun. If you check off both Sat and Sun, you will race both days.
GT races are not run on twin sprint weekends, but there are exceptions in the MW I think. In MA, like the Roebling Road TS, there is no GT race.
You can race HWSS, HWSB, ULSS and ULGP twice, once on Sat and once on Sun for a total of 8 races, just check both columns.
hope it helps