We used to have family reunions at least once a year, but it got to the point where nobody could agree on a time or a place. Some families already had their own vacations planned, and others had school or work obligations they couldn't get out of. Out of a family of 40, we'd be lucky if 10 of us could make it to the reunion.
I say if you want a better result, set a firm date and location for the reunion and just leave it up to individual families to decide if they can attend or not. There's no good way to accommodate every single family's schedules, so I say don't bother trying. If a formal family reunion doesn't happen, then try for a few private family get-togethers throughout the year.