There is no exact day that is ideal for every dog, but the average days that dogs can be bred while they are in heat are the ninth, eleventh and thirteenth days. The day that is ideal to breed will vary from dog to dog, but will often remain consistent from heat to heat in an individual dog.
Most breeds of dogs will go through heat around two times every year. During this time, they will cycle through four different phases. Proestrus is the first stage of heat. The hormones will just be beginning to change and the dog will be able to get pregnant. This is where the first show of bloody discharge will happen. During this period, the female dog will not allow any male dogs to breed with her. Estrus is the second part of heat and is the part that female dogs normally get pregnant in. The female dog will be fertile and will allow the male dog to mount her during this time. Diestrus is the beginning of a female dog's pregnancy, or the first stage that she will not accept a male for breeding. It lasts around 2 months in dogs who are not successfully bred. Anestrus is the longest period of heat, and is the resting period in between heats. It can last as little as three months or as long as 10 months.