So, here's the deal. We got engaged in December and our wedding is in Spring 2019. My FI's family is from a cultural tradition where big engagement parties are the norm and they are usually thrown by the brides family. They are typically like mini receptions and everyone brings gifts from the registry - I've been to a few of these and they're typically pretty OTT. My family is not from this tradition, but immediately got on board with the idea of throwing a low key engagement BBQ. We didn't (nor did my ILs) ask my parents to do it, but they just got onto the idea and loved it. My parents retired to a beach community / vacation town a few years ago and they love hosting and entertaining in their home, which, while not a large house, is basically made for this style of open house event. They typically throw 1-2 BBQs a year, including one on memorial day weekend, so they decided this year they'd up the ante on their memorial day BBQ and call it our engagement party - basically the same party they always throw, but hiring some simple catering instead of doing the cooking, a bartender, getting a nice cake, etc. and in addition to my family (who also vacations in this town), we'd be extending the invite to my ILs. ILs live a couple hours by car but the town we live in is near a destination they all go to often and regularly stay in hotels in this area in the summer. They say they'll travel for it, but I don't care if life happens and they decide against it. People keep asking us if we're having an engagement party and where we're registered (because, again, their family is all about these mini reception engagement parties), and we tell them "Memorial day weekend, bride's vacation home" and everyone sound on board. Given these events aren't typical in my family, we're definitely not expecting gifts or pressuring them, but if people wanna find our registry it'll be online. That's basically how we're viewing it.
Now, the wrench in the plans - my mom finds out she has to reschedule an elective surgery (it's elective but she really needs it, don't want to get into it much but just trust me that this is really the only option), and she basically will now be in recovery over memorial day weekend. She really doesn't have an opportunity to schedule her surgery whenever she wants it (i.e. right after the party) since she's faced with doctor availability and scheduling around other trips, events, plans that are important to her. She decides to have it on the new scheduled date and we immediately are like, "cool, ok, we'll cancel the party. ILs can throw us something if they want since it's their culture that cares about this, or nobody can throw us one and we'll be fine", but my parents are insistent they REALLY want to throw this party and can do it in August. We look at our calendar and there's only one weekend that is even possible in August, and it's the week before FI's cousins wedding.
I am *so* worried about thunder stealing or pissing off the future bride. She's a real sweetheart and is clearly super excited for us and didn't mind sharing "engagement thunder" with us at all over Christmas. BUT I am worried that having a party to celebrate our wedding right before she has her wedding will just be hurtful. We can't really have it after since after labor day the town basically shuts down, and it basically just changes the terms of the party. Plus, I am starting a grad program in September, and don't really want to put lots of plans on the early weekends. An invite is not a summons, and I basically will not be remotely hurt or offended if his family decides not to attend our event or send a gift given they will be attending the wedding the following week, and most of my family (obviously not invited to FI's cousins wedding) would be down for a normal summer BBQ, but I just don't want to hurt the brides feelings. The wedding is local to FI's family, so we're not asking for back-to-back travel.
I know I am techincally within etiquette and "you only get one day", but I really don't want to step on any toes. Would you be mad if you were the bride? I also know a lot of knotties would roll their eyes at an engagement party 6 months after a couple gets engaged, but again, it's just how it shook out. The memorial day party was about 11 months from the wedding and this will be 9.