My cousin (one of 6 girls on my mom's side) got engaged around Thanksgiving 2008. Set a date in Sept 2010. We got engaged in January 2009 and decided to get married Sept 6, 2009 (Labor Day weekend)
Literally as I was driving over to put down a deposit on our reception venue, my mom called and told my my cousin had switched her date to Sept 5, 2009 - the day before mine. Which is a hassle, but not that big of a deal except she lives 2 hours from our families and I live 4 hours from her. So essentially we'd have 2 weddings in 2 days that were 4 hours apart from each other. My family would not be there for a rehearsal dinner for me and it would require my almost 80 year old grandparents to drive all over the midwest all weekend.
She was having the wedding at her home, mine was in an upscale venue that ONLY had that date available at that price. There was no other date that would allow us to get married within our budget at the place we wanted, and there is a really limited number places like that around here. I emailed her to see what was up with the sudden date change - since we actually decided to get married in Sept 2009 partly BECAUSE her wedding was a whole year after that - and ask her if there was another date that would work for her since ours really couldn't be changed without throwing off ALL our plans. She emailed me back and basically told me I stole her date since she had already selected Labor Day weekend as HER wedding date (no matter that she had picked that weekend another YEAR) and she felt it was important to get married that weekend because her parents got married that weekend.
Her parents had a HORRIBLE divorce and her mom went off the deep end and they dont' even really talk...
Anyway, she did end up changing her date because the date didnt' work for her groom's father. But she was a total snot about it and obviously didn't care that I couldn't come to her wedding, neither could my sisters or parents, and she was totally inconveniencing the rest of our family - all because she switched her date after one had already been set.
Neither of us went to the others' wedding - she was on her honeymoon for mine and her wedding was the weekend before mine and it was too hectic to be out of town.