well, since i"m giessing you were not being monitored via ultra sound to know exactly the day of ovulation, (did you use an OPK?) really any of the days you had sex
you had the 28th-30th could have done it (sperm CAN stay alive for 5 days! some in perfect conditions can stay alive for 7 days!!!) & the morning of the 6th too. Your egg will stay alive for 12-24 hours AFTER you ovulate waiting around to be fertilized, so that day is very likely the day you conceived.
Good luck ;)