You can set Lessons to be locked until a given date.
Once you have a Course selected in the Courses and Groups dropdown menu, click the Lessons link in the Course Navigation bar.
Hover over the bar of the Lesson and click the Edit link.
Click the Lock until checkbox.
Set the Date and Time
Set the date and time the Lesson will unlock at by clicking on the day, typing the time in the time field, and clicking the Close button. Then, click the Save button to save your changes.