No it is not imperative that you have a date set. I know others on here will say how they had provisional bookings for rooms at hotels to have the reception, etc but you not have to provide them. Our only plan for our wedding was we were going to get married in 'May'.
I'm not a expert but I suspect this is going to cause you a issue as I believe the ECO isn't going to simply grant a visa to her as legally her daughter is actually her niece (guess you mean the daughter's aunt rather than your fiancee's aunt).