The Fairfax County School Board reversed the position it has held for four years and voted last night to observe the birthday of the Rev. Dr. Martin Luther King Jr. as a holiday starting in 1985.

The decision to observe the Jan. 15 date came as the board adopted a master school calendar to govern the number of school days and holiday observances in future years. Hitherto, the board has decided on its calendar each year, and next year's calendar--without the King observance--was decided on earlier this year.

Last night the board first rejected on a tie vote a move to add the birthday of the slain civil rights leader in 1985 and subsequent years. Then, when Centreville member Carmin Caputo said he intended to reintroduce the motion and juggled the proposed calendar so the King observance would not remove a school day (by reducing a proposed two-day end-of-semester student holiday to one day), several members made emotional speeches urging approval.

At-large member Robert E. Frye, the only black on the board, said, "It's hard for me to maintain my respect for the board members who opposed" the holiday. He said he thought the rejection "has something to do with race."

Wiping his eyes, Frye said, "For years people asked 'How ya doin,' Sam?' And Sam would dance and Sam would smile. Then there came a time when Sam stopped smiling and Sam stopped dancing and stopped saying he was all right.

"I realize this is Virginia and people think, 'Bob Frye, you shouldn't insist on this,' " he said.

Annandale member Laura I. McDowall told the board, "I grew up in Montgomery, Alabama . . . . While I was in college, Rosa Parks sat down on the bus . . . . Martin Luther King held a mirror up to our society and forced us to look at ourselves, and it wasn't pretty."

The vote was 6 to 2, with Chairman Ann Kahn and members Mary Collier and Joy Korologos joining Caputo, Frye and McDowall in favor. Members Anthony Lane and James Kitchin favored postponing for two weeks reconsideration of the first vote. Members Toni Carney and Gerald Phill were absent.

When Jan. 15 falls on a weekend, the holiday will be observed on a Friday or Monday.

In other action, the board approved a $3.3 million renovation project for W.T. Woodson High School.

Despite argument earlier in the year for postponing the project, which was funded in a 1981 bond referendum, the contract was approved without discussion.