You can say that someone "is available" if they don't have any other appointments or commitments at a certain time. Use this phrase to talk about people's schedules:

I'm available on Wednesday after 3:00, or any time on Friday.

When will you be available?

When someone is applying for a job, the date that they are "available" is the date when they no longer have to go to their previous job:

I'm available immediately.

I'm available starting next Monday.