Generally about 2 weeks BEFORE when you expected your period (the one you missed because you were pregnant). If this is a question of paternity, you'll need a blood test. The dates of 6th, 9th and 13th are all quite close together and if sperm can live in a body for up to 6 days... the dates overlap.

I would say Oct. 6th.

How can we know? She didn't say she was regular or irregular. She didn't say when she expected her period.

She'll still need a blood test if it was a different person on the dates of 6, 9 & 13. Those days would overlap with possible ovulation and sperm life.

exactly. there is no way anyone can know the exact day of concpetion.

Well, I didn't say
a) I'm a doctor
b) this answer is written in stone

What I did say was, "I would say..."