A North Carolina broker with RealtyOne is working for a buyer client to find a...

A North Carolina broker with RealtyOne is working for a buyer client to find a home. They find a home listed with RealtyTwo and submit an offer on Wednesday afternoon. The listing agent presents the offer to the seller on Thursday. The seller signs the offer with no changes and returns it to the listing agent on Thursday evening. Friday morning, the listing agent calls the RealtyOne broker and states the seller has signed and accepted the buyer's offer. The RealtyOne broker receives the signed documents on Saturday. On Sunday morning, the RealtyOne broker meets with their buyer client, informs them of the acceptance, and delivers the signed documents to them. When did the parties form a valid and binding contract?