In the state of Washington, if an employee submits written notice of resignation, then changes their mind the next day and writes another letter retracting that resignation, is the employer required to accept the retraction?

No they are not

Can you give me an idea of where I can find hard copy giving me the details on that?

There isnt because they exist no law that forces an employer to accept your recinding of resignation. They could even decide to terminate you if you wont resign.

There are a lot of things that are legal because no law says they are not legal. This is one of them.

The employer is not required to accept your retraction of your resignation because there is no law that says they are.

Sorry... no do overs in labor law.