You can choose to set a start date for your quizzes. You can do this on the Set Quiz Dates page when setting a test.
If you set a start date for a quiz, the quiz will show up on students' My Educake pages at midnight of the date you selected. For example, if you select 22nd September as a start date, students will be able to see the quiz starting at 00:00 on the 22nd.
If the quiz as already been set, you can click on its Start Date to change it.