After stirring up a cauldron of controversy, the Fairfax Board of Supervisors has changed its mind and decided to let the witches and goblins of Halloween reign in the county this Friday.

Ratifing an informal decision made hastily last week after a storm of complaints, the supervisors decided yesterday not to move back the county's celebration of Halloween from Friday (Oct. 31 -- All Hallows Eve) to Thursday.

The change had been routinely approved after a request by the Fairfax Police Department, whose officials were concerned that a celebration on Friday -- when traffic is heavier -- would create more hazards for children on their door-to-door trick-or-treat rounds.

The decision had the supervisors' phones ringing off their hooks.

Board chairman John F. Herrity (R) said one of his many callers asked, "Why does the board have to change holidays in order to exist?" Another call came from a young boy, who asked, "Are you the one that did away with Halloween?"

Just before the supervisors changed their minds yesterday, board member Nancy K. Falck (R-Dranesville), defended a Thursday celebration for Halloween.

"I can't think of any greater danger to little ones," she said, "than to get out on a road after dark with all the teen-agers going to football games."

Supervisor Marie B. Travesky (R-Springfield), who supported Falck, said, "I just think we're all going to be praying Friday night."

The clincher was provided by Supervisor Sandra L. Duckworth (D-Mt. Vernon): "The concern of many teachers and PTA parents was that the children were going to go out both nights," regardless of what supervisors originally did.