When is it safe to have intercourse?

If what you mean by "safe" is avoiding pregnancy, the only safe time is long after menopause is complete. Seriously. Those dangerous fertile periods vary from woman to woman, and even in one woman they vary month to month. It's also variable how long a man's sperm will live and lurk waiting for an egg to appear, depending on how hospitable the woman's chemistry is to the little swimmers. There's an old joke: "What do you call couples who practice the rhythm method?" Answer: "Parents."