Passport section 4

While filling passport form, section 4 expect about date of marriage of father. In my scenario, this information is not known. Can I leave it blank?

I think it is needed. If you cannot ask your parents or if they have forgotten as well then you can write reason in section 8.

I knew the year they got married but not the exact day so I put first of that month.
They asked me that question on passport interview.

Yes you can leave it blank.
If the date is not relevent to your application then there is no need to write it. I was in the same situation and I simply left it blank, did not explain anywhere and it all went smoothly, they did not say a single thing in the interview or anywhere else about it.