Ask a landlord-tenant lawyer and get answers ASAP
I'm Lucy, and I'd be happy to answer your questions today. I'm sorry to hear that this happened.
You unfortunately do not have a legal right to retract your notice, because you and the landlord agreed to terminate your contract on 12/20. To remain living in the apartment, you would both have to agree that you can stay. The problem is, the landlord has already rented the place to someone else. He could get sued if he is not able to provide them with a place to live on 12/20. What you CAN do is talk to the landlord and explain the situation, see if there's another apartment he can put the new tenants into instead. But if the landlord won't agree and you choose not to leave, the landlord can actually sue you for money he has to pay the new tenant when they can't move in (which could include hotel costs, storage fees, and moving costs).
I apologize that this was probably not the Answer you were hoping to receive. However, it would be unfair to you and unprofessional of me were I to provide you with anything less than truthful and honest information. I hope you understand. Please rate my answer positively to ensure I get credit for the time I spend helping. If you are on a mobile device, you may need to scroll to the right. Thank you.Good luck.