
There is a "window" of roughly 5 days during a bitch's season when she is fertile and will be willing to mate. On average, the best day to mate them is day 11, but all bitches are different, and the best day will vary not only from bitch to bitch, but also from season to season with the same bitch!
When a bitch is "ready" she will whisk her tail to one side if you rub your hand firmly on her back, near her tail. She should also brace herself, as she will need to stand strong and steady to receive the dog - the signs are unmistakeable when you've seen them once! :)
Hope this helps!