If you have all your course lessons marked complete, a check mark will appear at the top of the page.
Click on that to move the day number forward and reset the page for the next day.
If you want to mark the day complete but not all assignments are complete, go to the Settings page (Parents link), and click on the check mark by the day number at the top of the page. Do not mark the green check mark from both the Student page AND the Settings page. You only need to mark the green check mark on the Settings page if your student did not complete all subjects for the day.