Ahead of each fixture, your team admin can ask the whole squad for availability. You'll see the request against the upcoming match. Respond as soon as you know, so your admin has accurate numbers while they're planning the squad.
Your response doesn't automatically decide who plays. Team admins use everyone's answers as context alongside their own judgement when picking the squad and line-up (see Running matchday). If your plans change after you've responded, you can update your answer any time before the match.