If im using the rhythm method of family planning when should i avoid intercourse in a 28 day cycle?

All the time: There is no safe time. If you want to avoid pregnancy, use contraception.

28 days: You know what they call people who use the rhythm method? Parents! rhythm method is like the Aspirin method of birth control (keep an Aspirin between your knees to avoid pregnancy). Anyway, if you must, chart your cycle extremely accurately and find your ovulation time. You are most likely to become a parent 4 days before and 4 days after ovulation.