Purchases triggered by the tasks's due date

First of all, you will need to set up tasks to trigger purchases. To do so, you can access them by opening a workflow. Workflows are accessible through Setup > Processor > Workflow in the menu or from Scheduling > Maintenance > Workflow.

You can now configure one of the tasks to trigger the purchases for the Just in time. You can do so either in the Workflow management detail table or directly in a task.

Note that only one task can be configured as such per workflow. Changing it will affect the dates for the purchases.