You can choose to set a start date for your tests. You can do this on the Set Test Dates page when setting a test.
If you set a start date for a test, the test 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 test starting at 00:00 on the 22nd.
If the test as already been set, you can click on its Start Date to change it.