
The ideal day to mate would be three days after ovulation (when all the most eggs will have reached maturity as ovulation takes about 24 hours to complete), this is what would be done if the bitch was having AI where the sperm lives for such a short while compared to fresh natural service.
You also have to take account of likely receptivity, some bitches and especially some males will not mate a bitch until she is really ripe, even though she could become pregnant from an earlier mating.
Certainly in my breed both young and experienced males will often not want to mate until 2 - 3 days after ovulation.