http://www.germantownpreschoolfair.com/ is also on Wednesday, in the evening.
Note that the Wheaton fair is actually a little bigger than the list of schools on their website makes it look- I know my school (Rockville Community Nursery School) signed up a couple weeks ago but they must be behind on updating their online list.
MOMS Clubs of Gaithersburg and Rockville/North also did one together in January last year but I don't know what their plan is for this year. Maybe someone else knows?