You can have a leaderboard be active only a part of your entire event duration. E.g. a weekly or a monthly leaderboard within a longer event.
To configure a leaderboard for a shorter time frame you can Edit the leaderboard and then set the active from and active till dates in the Activity Filter section.
Please note that your leaderboard cannot start before the event itself starts or ends after the event itself ends.
Optionally, you can change the visibility of your leaderboard to Auto Mute so that it will only show during the active period and will be hidden when not active.