There's two types of Science Fairs (Computer Team and Chess Team) as there are two types of "Sports Day".
Unfortunately the Science Fairs need to be scheduled by one of the members of the team. They do not happen automatically. From what I gathered from pre-release information this is "as designed".
When you schedule a fair, you schedule it for the following day. If there is a conflict with another fair the option is disabled.
I had a Chess fair on Tuesday and now I have a Computer fair set for Monday. Each was scheduled the day before (you can schedule on Sunday).
For what it's worth, I also think it was badly done but hopefully this will allow you to hold the Science Fair.
