Hi
Obviously at the end of the day when you have it done is your choice.
However in my opinion the best time is half way between the first and second season, as this avoid the possible complications that can occur if done before the first season.
I have just done a quick search on the net, and found this article which may be of some use :
spaying your dog.htm
I hope this is of some help, but is only my personal opinion, and you should consult your vet before you make and decision based on this information.
Many Thanks
Pets Pantry