To override or change the service requirements for one or more weekends of the month, you can use a monthly service.
Before creating a schedule, go to the Service Times pane inside of MSP and click the "New Monthly..." button to the right of the pane. Set this service up to occur the 2n1st weekend of the month at 9 AM, for example, and then set up the required ministries for that service.
Make sure in this window to check the, "Replace coinciding weekly service" box, and this way when you generate a schedule, the 9 AM service on the 1st week of the month will be replaced with this monthly service.
This is a great way to set up your schedule if you have different requirements multiple weekends of the month, too, because the services will all be "treated as" the regular Sunday 9 AM service, so you won't have to have multiple service preferences for your volunteers.