If you would like your students to be able to participate in the course prior to the official semester start date or allow students to participate after the semester ends, you can change the start/end dates of your course.
Select settings in the course
Select Course Details
Set "Participation" to "Course" and it will allow you to change the dates.
You also have options to restrict students from viewing the course before/after the start/end date.
Select "Update Course Details" at the bottom of the page to save your changes.