whilst in season, which day is best to mate with a stud dog?

when the bitch is ready:)
Only joking. If you are unsure take your bitch to vet from about 10 days and have her tested, he will tell you when she is fertile and that gives you a couple of days after to mate her, as she will remain fertile for a few days. mari

Most staffordshire breeders say the 12 and the 14th day since the dischage became red. I mated mine on the 13th + 15th day as she had a longer season than most. Other signs are : tickle the back leg where ir begins to bend and her tail should go to the side, or another is when the discharge turns from a red to a staw like colour - though this last does not always happen. Or as mari says take her yto the vet, I did this anyway with my bitch, when i knew she would stand i took her in to check i hadn't gone over.
As this is obviously your first breeding I hope that your bitch is on her 3rd season or later and that you have done all the relevant reading ,etc, to understand what your taking on - this comes from someone who is perpetualy woried about her bitch, and the whelping, and the puppies, and she still has another 3 weeks til her due date!!
You can mail me personally if you would like information etc, from someone in the similiar position to you.