"I know your sons aren't ready for Kindergarten yet, but I trust your opinion so here goes:
My sister lives in another state and she's getting married this year. She wants to get married on Labor Day weekend. My sister wants my daughter to be her flower girl. Unfortunately, my daughter is starting Kindergarten this fall. School will probably start the week before Labor Day, so I'd have to take my daughter out of school at least one day if not the whole week to be at the wedding. (We would be driving to the wedding because we can't afford to fly, and the trip is about 13 hours of driving, so 2 days of driving.) I'm not willing to do this because I think that my daughter will miss out on the friendships that occur on the first few days of school. Do you think I'm overreacting? I feel bad for making my sister choose a different date for the wedding, but I also need to be an advocate for my daughter. My daughter goes to daycare/preschool everyday because I work out of the home, so I know my daughter will not have problems adjusting to being away from me. I also don't think she will have problems making friends at Kindergarten, I just want to make sure she has every opportunity to do that."
Thanks for the vote of confidence--I hope I can live up to your expectations.
I'm going to completely leave your daughter out of it for a minute, and suggest that your sister should not have her wedding on Labor Day to begin with. It sounds like a great idea to have a wedding on a holiday weekend, because everyone has that day off so they can take an extra day after the wedding. But in reality, you're costing them more money because all plane tickets are super-expensive on a holiday weekend, and hotels are more expensive. Plus, you're robbing them of a holiday weekend they could otherwise spend going somewhere else or just sleeping in and drinking beer and barbecuing in the backyard or doing whatever they like to do best. Which is most certainly not going to a wedding, no matter how much they love you and want you to have lifelong happiness. (Unless they really love the Electric Slide.) So it's actually a favor to your guests not to have your wedding on a holiday weekend.
You know, it just occurred to me that if you could convince your sister not to have the wedding on Labor Day weekend for the above reasons, then your daughter could be completely left out of it. Which would just make it easier for everyone involved.
The actual issue of whether or not missing the first few days of school is going to hurt her socially is a tough one. It's absolutely true that she'll be able to make friends even if she misses a few days at the very beginning of school. However, it's also true that missing time in the first week will put her at a disadvantage and might make her feel insecure and confused. I don't know your daughter's personality, either. Some kids can just hop right in and not feel self-conscious. If it had been me at that age, missing a few days at the beginning of school would have made me even shier and hesitant than I already was (with the other kids--with grown-ups I was fine). Kindergarteners can be tough.
If you think it's going to make it awful for your daughter to miss those days, then you have to go with your gut. But honestly, I'd try to see if your sister will move the wedding date (although it's probably a little late for that by now since it's only 8 months away) just to make it easier for her guests. If that works, then you won't even have to consider the issue with your daughter.
Let us know what happens.