If you need to run a schedule and your mass times or required ministries will change within that schedule, you have a couple of options:
Create multiple schedules
The fastest way to create a schedule with different mass times is to create two separate schedules: one with all of the services before the change, and one after. This will allow you to run one schedule with the first set of requirements and then edit the Service Times pane before running the second schedule, saving you from having to manually make any changes once the schedule is generated.
Create one schedule and make adjustments
If you do need all of the times on one schedule, the best option is to run the schedule with the predominant mass active.
Before you click the "Commence Scheduling" button, click on the days where those services are not needed, click on the service, then use the "Delete Service" button to manually delete the service from that particular day.
Use the "Add Service" button, making sure the "treat as" drop-down is selected for the correct mass. You can then copy this mass and paste it into the other dates that would have the old times. Be sure to edit the "treat as" to the correct service time each time you paste so that volunteers are scheduled correctly.
Now you can use the "Commence Scheduling" button to fill your mass times.