Go to Officials and click Edit Availability under the actions column.

Availability:

These are the dates the official will be available through the set date range for each day selected.

For example, someone who can only officiate the month of December 5:00pm-8:30pm during the week and 10:00am-3:00pm on Saturdays will have an availability like this

Availability Overrides:

Add or remove a specific date of availability from their current availability. Working from our current example, lets say they can work the last Sunday of December.

Blackout Dates:

Specific dates that the officials do not want to be scheduled to officiate or work an event. This may commonly be holidays.

Officials can also leave notes about their availability that both your association and the official can view.