The Montgomery County Council voted unanimously yesterday to observe the birthday of the Rev. Dr. Martin Luther King Jr. as a county holiday on Jan. 15, eliminating Maryland Day as a holiday.

The vote got brief applause from about 20 black county residents, many of whom had lobbied intensely for a county holiday commemorating the slain civil rights leader.

"I'm delighted with this," said DeVance Walker, minority affairs assistant to County Executive Charles W. Gilchrist. "It puts the county in a more progressive light. I feel like going out and celebrating."

About 5,000 county workers now will not have to report to their jobs on King's birthday. If it falls on a weekend, as it will in 1983, the council will decide whether to declare the following Monday a holiday. Initially, some employes had opposed another holiday so close to Christmas, New Year's Day, and Washington's birthday and removing a holiday in the early spring.

Maryland Day, March 25, commemorates the landing of English settlers in the state in 1634.

Several council members voiced concern that King's birthday might become as commercialized as the holidays honoring Washington and Lincoln, which they said have been marked less by reflection and observance than by department store sales and fireworks.

Council member Neal Potter called the King birthday "a holiday of the highest relevance to our time," and added, "I hope we continue to look for lessons and not for sales and celebrations."