You can configure a leave type to require events to be submitted for approval X number of days before the event start date.
To do that, from the Dashboard, choose the Admin menu and click Event types and allowances.
Then select the event type you wish to set up and click Edit Event type.
Choose the Restrictions section from the sidebar on the left. This will display a screen as the one below:
Note the Request Leave in Advance option. Make sure it is selected and enter the appropriate number of days.
When ready click the Submit button at the bottom of the page.
When Employees request leave of this type in the future and they have selected a date that is fewer days in advance than the configured date they will get a warning message like the one below: