You can preassign a volunteer for one particular date by manually  adding the assignment to the schedule skeleton before clicking the "Commence Scheduling..." button.

1. Go to the Schedules pane in a VSP installation, enter the start and end dates for your schedule, and then click the "Create..." button.
2. Select the day on which you'd like to make the assignment, and then double-click on the position.
3. Select a volunteer to fill that position.

You can also go to the Preassignments pane in VSP if the date is for a not-yet-created schedule.  

1. Click the "New Preassignment..." button in either a VSP installation or in the Admin view of the Web Terminal and create the desired position.
2. Leave the settings as "Every week" and "Every month of the year", then enter the desired service date in both the "Starting" and "Ending" date boxes.
3. Click "OK".
4. When you do generate the schedule that covers that time range, this position will be included.