Do you want to ensure the volunteer always serves every other weekend, or just that the volunteer is available to serve every other weekend, but they don't always need to serve every other weekend?
If you want to ensure the volunteer always serves every other weekend, you will want to use Preassignments to set this up.
- In the Preassignments pane, click on "New Preassignment...".
- Select the option for "Every other week" starting with a specific date.
If you just want to make the volunteer available to serve only every other weekend, but not to mandate that he is scheduled every other weekend, then you will want to use "can't serve" dates to make the volunteer unavailable every other weekend (starting with the weekend before the volunteer would prefer to serve).
- In the Volunteers pane, select the volunteer and then click the Edit Volunteer button.
- Then click the "Add..." button next to the "Can't serve dates / times" field and choose "Every other weekend".
- Then select the first weekend on which the volunteer can NOT serve as the starting date for the "can't serve" time.