Login as a user with administrative or manager rights and go to Manage -> Locked periods.
The existing locks are shown in the left panel, ordered by start date.
To create a new lock, modify the start and end date under "new lock" tab. Everytime you change either the start or end date, the name is updated to reflect that change. You can always pick your own of course.
You can exclude (eHour 1.4) users from the locked period. Click on add/remove users to select the users you want to exclude from the lock.
When you're done, press "Lock" to lock that period for all users.