Looking back I am so glad that I decided to organize a math fair at Wolfville Junior High School this year. I can say it definitely blossomed from the beginning. We went into it thinking there would probably be about 15 projects in total and would be something small for students to show of their work - well it definitely grew since then. We had almost 50 projects including those from grade nines, and sevens in the school as well as a few projects from Acadia Education students with a teachable in math.
I can say that it is definitely a lot of work and take any help that you can get when organizing the fair, whether it's for judging, setting up, taking down, or even coming into your classroom to help out students with their projects. Make sure you have support of other teachers (in particular the other math teachers) in your school so that everyone can benefit from this event.
Start your planning early. It never hurts to have students working on the projects for an extended period of time (whether working on their projects inside or outside of school). Have as much help the day of as you can, from experience I can tell you I was very tired the day of the fair (from setting up the day before and from getting everything prepared the week prior).
If you are a teacher planning on organizing a math fair I would suggest that you look after your own students in terms of projects and collaborate with the other math teachers so that they look after their students' projects. It is definitely enough work to organize the fair alone that the more the other teachers can help the better.
Overall, the math fair was a huge success. We have students asking us if we're planning on organizing a fair another year. It was great for the students throughout the school to have access to the projects to interact with and see where math can take them (and sometimes to places that they never even thought about). The students really enjoyed working on their projects and commented that they enjoyed working on them and that they got credit for something that they had fun doing. So, if you are at all interested in organizing a math fair definitely look into it farther and talk to your administration at your school to see what support you can get for this endeavor. I think your students will really appreciate the end result and enjoy math better for it.