"The game is not on Saturday."
Translation:El partido no es el sábado.
The answer is saying the game is only on "the" Saturday of this week; not every Saturday.
Instead of using "on" for days Spanish uses "el." If you want to say every Saturday, it would be plural "los sábados"
Without "el" it sounds to the Spanish speaker like you are stating the the game is not ONa day of the week, but it actually IS a day of the week.
Ser is always used for date/times and events. These two mnemonics really help me:
SER: DOCTORE (D-date; definition, O -occupation, C-characteristics, T-time, O- origin, R-relationships, E-events
ESTAR: PLACE (P- position, L- location, A-action, C- condition, E-emotions.