Oh, yes. fertig is primarily 'being done'. I'm done with my homework – Ich habe meine Hausaufgaben fertig. 'bereit' is something you're prepared vor. Bist du bereit für deine Rede? – Are you ready for your speech? Alle Maschinen startbereit. – All engines ready to start.
So, 'fertig sein' is having something completed, finishing a task or meal, being done with something. They have the common meaning of being ready when you prepare yourself to leave. Do you have your purse, are you ready? I really want to leave now. Here, ready would very likely be 'fertig' in German. This might be an idiomatic use though. I can't come up with any other example than that.
Some~thing is simultaneously Fertig (regarding the PAST actions that made the thing be what it is now) and Bereit (regarding the FUTURE actions involving the thing).
The moment the homework is ready in English = the homework is both Fertig (done by the student) and Bereit (ready to be shown to the teacher) in German.
On a timeline Fertig and Bereit would both stay on the same mark, which is the frontier that separates the past from the future. If the frontier would be a wall, then on the side facing the past would be written Fertig, and on the side facing the future would be written Bereit.
I guess in English (not a native) it would normally be written Ready on both sides.
Thank you so much, this explains everything!
I've heard them both and have wondered about that.
For example, in Kommissar Rex, Moser says "ich bin gleich fertig" = "I'm almost ready" (with a meaning of "done" in this context) when Stocki gets to his place and he's still in the shower.
But when seeing Kreator live in Germany, Mille asks the audience "seid ihr bereit?" = "are you ready?" (with a meaning of "prepared" in this context).
Now it finally makes sense.
One is (hopefully) "bereit" at the beginning of an event, and "fertig" at the end of it. "Bereit" means "prepared" (to go) while "fertig" means "finished," or "done" with it. That is to say that "bereit" means "ready" in the sense of prepared, and "fertig" means "ready" in the sense of complete.