Candidate Availability is indicated in the Contact record on the General tab using a Date field.
In Mercury Legacy Search you have the ability to filter Candidates by Availability. After you've created your search criteria or used a saved search, click on the Filter icon.
In the Availability field choose a date and click Confirm
Your results will then change based on Contact records Availability date indicated. Your new results will show all Contacts whose availability date is on or before the date you selected.