Go to the page you'd like to schedule the active time for
Open the actions menu and choose 'Page settings'
Open the Advanced settings
Choose the option Schedule when page is active (not in read-only mode)
Choose the time frame during which participants can contribute - other times, the page will be in read-only mode (participants can access the page but cannot comment, answer to polls, etc., before or after the chosen dates)
Administrators can edit the page settings at any time.