I feel a bit differently about the sitch.
May 15 and June are sooooo close. These people are going to be your family FOREVER and EVER and EVER and EVER. (did I say FOREVER?) <<Well, it is FOREVER. This is such a simple thing. Waiting a couple of weeks isn't going to hurt anyone.
I mean it is your day.......I totally understand. But if this day isn't totally set in stone, with all of the deposits down and everything, I would just APPEASE (prob. sp error) them. Graduation is important... AS IS A MARRIAGE... but this is your soon to be Mother in Law. They just don't and won't understand something when their minds are set on something.
You could go ahead, do the wedding, and not have her be a part of it even though you might want her to. (and i am sure she wants to) OR just wait, look like the "good girl" (for both your fience' and her) and live happily ever after.
NOW........If you totally DISPISE her
and really don't want her to have ANYTHING to do with your flowers at ALL.......... Keep the date.
Just my honest point of view.
Not that it means anything.
Just another p/o/v
A blessed weekend to you