When is the safe day to have sex after menstration?
Since your are looking for safe times to have sex, I must conclude that you are hoping to avoid pregnancy. Since most married couples are usually hoping for a pregnancy, then I have to assume that you are planning on committing fornication. Having sex when you are not married is a sin. "Marriage is honorable among all, and the bed undefiled; but fornicators and adulterers God will judge" (Hebrews 13:4).
Every act of sex contains a possibility of pregnancy. The question really is what odds you consider to be "safe." Pregnancy occurs if a sperm reaches an egg on the day that it is released. A girl releases an egg 14 days before her next period starts. However, a boy's sperm can last up to six days inside of you. Therefore, if you have sex within six days of when your egg is released, you could get pregnant.
Complicating matters is the fact that menstrual cycles aren't always consistent. While the average is 28 days, it can be as short as 14 days and as long as 36 days. Even a girl who has regular periods could have an unexpected change one month. Since the only firm number in this cycle is that your egg is released 14 days before the start of your next period and you can't say with absolute certainty when your next period will start, it makes guessing when unprotected sex won't lead to pregnancy difficult.
While everyone focuses on pregnancy, there are other dangers, such as sexually transmitted diseases. These can be transmitted at any time. While we have medication for some of these diseases, many of them have no cures.
Rather than take risks to rush a sexual relationship, you are safer to wait until marriage to have sex.