You can preassign a volunteer for just one particular date by adding the assignment manually to the schedule skeleton before clicking the "Commence Scheduling..." button.
- Go to the Schedules pane, enter the start and end dates for your schedule, then click the "Create..." button.
- Click on the day you'd like to make the assignment, and then double-click on the position you'd like to fill.
- Select a volunteer to fill that position.
You can also go to the Preassignments pane in MSP if the date is for a not-yet-created schedule.
- Click the "New Preassignment..." button to establish the position for which they should be scheduled.
- Enter the same date in the starting and ending date at the bottom of this dialog.
- Click OK.
- When you do generate the schedule that covers that time range, this position will be included.