Why is it 高考那天, not 高考的天?
天 in 那天 refers to day, so 高考那天 means the day of 高考.

天 could refer to sky too, such as 高考的天 (and 高考之天), which means the sky of 高考 and doesn't make sense. (It's same for 那高考的天, which could be literally translated as that sky of 高考.)