Why did we take the college entrance examination twice in 1983?

I took the college entrance examination in 1983. There were not two college entrance examinations that year. However, in those years, there was a pre-examination before the college entrance examination, and about half of the people had to be eliminated. Because there was great pressure to change the papers for the national unified college entrance examination, the pre-examination was required to reduce the number of people taking the college entrance examination. Another thing that is very special is that the college entrance examination in 1983 was held on July 15, 16, and 17, which was different from the previous July 7, 8, and 9. I told many people, but no one believed it. , some people even laughed at me for taking the supplementary exam to get into college. This is my personal experience and cannot be mistaken.