When should I avoid sexual intercourse when I am using the natural family planning method?

Around midcycle. Get an average of your cycle length over 6 months. The day your period starts is day 1. You can calculate when you ovulate by subtracting 14 from your cycle length. If you have a 28 day cycle, you will ovulate on day 14. If you have a 30 day cycle, you will ovulate on day 16. Avoid intercourse starting 4 days before ovulation to four days after ovulation.
Midcycle. If you have menses every 28 days you should ovulate on day 14 (days from the first day of your period starting). Eggs are viable for about 36-48hrs and sperms for about 48-72hrs. So you should avoid intercourse, at least unprotected, 2-3days before and after mid cycle. 5-6 days total.