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Certainly the bride/groom have had some mis-steps in how they are planning things, such as guests needing to bring their own chairs. And, although it is not an etiquette faux pas to have one's wedding on a major holiday weekend, couples who do that also need to understand it could make attending more difficult for their guests.
It's really up to what you all want to do. If the inconvenient day, due to lack of parking and perhaps general mayhem at the park, overrides your desire to see this couple getting married, then RSVP your regrets. Or just the long roundtrip drive itself...especially in July 4th weekend traffic. An invitation is not a summons, though I understand where it can be harder to do that with people we care about.
OP did you delete your post? I gather it's outdoors over July 4th weekend? I'm not a fan of outdoor summer weddings so I'd lean towards no, since I hate the heat. Unless it was very close friends. Can't really comment further because I don't know the circumstances.
It gets better.....I just called the park, there's NO parking. So........
Thanks for the update! Honestly, their wedding sounds exactly like anxiety dreams I had before my own wedding, where the wedding day came and nothing had been planned. Congrats to the couple, but it sounds like you made the right choice to stay home!