If you are using Time Card Approvals your pay periods will not lock or unlock automatically. The Time Card Approval status determines if a time card is locked. Here are the various statuses for a time card and the associated Lock/Unlocked status of a pay period. Locked means time cannot be entered:
Draft = Unlocked
Pending Approval = Locked
Approved = Locked
Rejected = Unlocked
If you are not using Time Card Approvals then your pay periods will lock automatically once the pay period ends. However you can manually lock and unlock them as an administrator. Go to Settings --> Pay Periods. From here click on "Edit" next to any pay period. You can lock it for just employees or lock it for all users. Administrators can lock/unlock specific pay periods at any time.