For many elementary school educators science fair time is just one big headache. There are different approaches to helping your students prepare for the fair.
Some educators like to start in the fall and work on the projects throughout the year.
I personally like the science fair to be in one block. All the students in my class prepare a project either individually or with a partner. We work on these projects during Science class and Language Arts. Scienceworld has an encouraging home page with good reminders about why we are doing these projects in the first place. The Teacherlounge offers some good advice for organising the actual science fair and some project ideas.