Each worker is in a separate field (Worker1, Worker2, Worker3, etc.).
Not a good idea. Use separate records in a related table for each worker, not separate fields. You may want to check out the Event manager starter solution that comes with Filemaker or this demo file for ideas on how this might be done: http://www.4shared.com/file/dZ0bjclw/ManyToManywDemoWExtras.html
Please note that the demo file is not intended to be a full up demo on event management, but since it uses event management as the model fo rmany to many relationship interfaces, it still has some techniques you may find useful.
A validation rule using a relationship that matches by event date and worker name could be used for you fields, but this would require separate relationships for each of your worker fields. That can be done, but it produces a very complex design that will be difficult to work with.