You can assign dates to each unit. This will allow you to close off information in that entire unit to students until the selected date and time.
Step 1: In the navigation tree, click the Author tab and the Unit for which you want to schedule and assign dates or deadlines.
Step 2: Click Toolbox.
Step 3: Click the Edit Schedule button.
Step 4: Use Course start and end dates will be the default. To change this, select Use the following dates.
Select start date and time. Make sure you check the box Restrict access before if you do not want students to have access before this date.
Select end date and time. Make sure you check the box Restrict access after if you do not want students to have access after this date.
Assign other preferences accordingly.
Step 5: Click Save & Close.