Fixed retainers are fitted straight after the braces are removed (and sometimes just before), at the same appointment. Removable retainers take longer to make. The earliest you can have them is a few hours later on the same day, but more often you receive them the day after your braces come off. With aligners, once the attachments have been removed from the teeth, the last aligner can be worn as a retainer for a while until a proper retainer is made.
This answer is for general information. Your own case needs an examination by an orthodontist.