There's a few things to consider when planning a reunion:
- Scale (who to invite)
Second is cost. Family reunions take money, how are you going to figure out how much to spend and how to collect it from the attendees? On figuring how much to spend, a good rule of thumb is to use the median income of the attendees, rather than the average (averages can be skewed too much by outliers). That is, if most of the family is lower-middle class, plan for that range, ignoring the 'rich' uncle -- don't expect him to foot most of the bill. The bigger the scale of the reunion, the more you will have people that would struggle to get there and the less people will care about going, so keep that in mind as well. I have found that there are a few things that help a ton, one being family trust funds (i.e., when great-grandpa dies he leaves money in an estate especially marked for reunions) and the other being serious advanced planning. I've had reunions where we calculated the costs, divided by attendees, and collected monthly amounts per person for over 2 years. That works well, especially if you round up on what is needed.
Location is easy if most of your family is in the same area. Have it local. But if your family, like mine, is spread out all over the country (or the world), then you have a tough time. Do you plan according to the location of the organizer, or do you try to do someplace central, or someplace with sentimental value? I really can't answer that, because it depends on the budgets and tastes of your family. But if you do it according to the location of the organizer, you MUST switch organizers on a regular basis to make it fair to the outliers.
Schedule. We've found that reunions every 2 years is usually a good amount, especially if you can pass off between families every year (e.g., Mom's family this year, Dad's family next year). If the scale is BIG, do it less often, if it's small, you may want more often. As to time of year -- if there are kids in school, you have little choice but to have it in the summer when the most people are available to come. But if you're all empty nesters or home schoolers, try having it in the off-season. It's fun, open, and much cheaper! Whether you plan around a holiday is up to your family, with pros being that people get an extra day off work but cons are that people often have other plans on holidays.
Activities. Keep activities varied according to ages, with the younger kids needing more planning. The older your attendees, the fewer activities you'll need to have -- family members getting together often just want to sit around and visit. Also, try to cater to the tastes of the group -- I wouldn't necessarily plan a camping trip for a bunch of computer programmers, for instance.
Don't worry about having EVERYONE there. Same as with life in general, you can't please everyone, and not everyone is going to be able to make it to your reunion. Try for the majority and let the rest go. Make sure to take lots of pictures. Wear sunscreen. And have fun!