When is the right time to have sex so that we are assured of a pregnancy?

Around ovulation. There is nothing that will assure a pregnancy. The best time to try is around the time of ovulation, or best shortly before. For most women this is around mid-cycle, women with irregular cycles may be harder to pinpoint. Counting cycle days, following temperatures, home opk are often helpful. Fertility specialists and some ob-gyn's can help you time this very precisely.