If decorating the family Christmas tree is an annual labor of love, consider what it takes to get the National Christmas Tree ready and to kick off the attendant Pageant of Peace: Simply labor.

Tonight around 6, President Clinton will flick the switch that illuminates the National Christmas Tree, a tradition dating back 76 years on the Ellipse. But that's the easy part; the real work began months ago.

It was early March when Kathy Presciano, a lighting designer for General Electric, started sketching a concept for the big tree, marking the 37th year that the giant corporation has donated lighting and decorations for the evening's major star.

Next, Pageant of Peace consultant Audrey Hartzell solicited the 56 states and territories for special ornaments to decorate the smaller trees that line the Pathway of Peace encircling the big tree. Millennium themes were welcome this year, she wrote, and students from Crofton Middle School in Anne Arundel County responded promptly with colorful "Y2K Bugs" featuring big bulging eyes and stick antennas.

Entertainment gurus Robert M. Johnson and Joe Fab of the McLean-based Johnson Group began the search for tonight's talent, culling through lists of popular headliners. Hopes for Ricky Martin and a Living La Christmas Loca show faded because the Latin pop star was too busy. Finally, they landed Mr. Las Vegas, Wayne Newton. He will be joined on the program, which starts at 4:30 p.m., by opera soprano Renee Fleming, country western singer Marty Stuart, cast members from the musical "Chicago," the Urban Nation H.I.P. H.O.P. Choir of the District, the U.S. Marine Band and NBC "Today" weatherman Al Roker--playing Santa Claus.

By summer, National Park Service horticulturist Helen Matthews was diligently dusting the branches of the big tree--a 39-foot-9-inch tall Colorado blue spruce--looking for tiny red spider mites that spell death for evergreens. It then received its once-every-three-years dose of fertilizer, a bit of pruning and extra water because of the drought.

In October, National Park Service workers began laying roadways and boardwalks in the Ellipse's President's Park. And in early November, Ron Hudler and his North Carolina tree farm crew cut the 56 eight-foot-high fraser firs that he donates each year as state and territory trees. They were trucked from Appalachia to Washington.

Then, three weeks ago, Ken Bisch and his crew from Hargrove Inc., a special events and trade show company in Lanham, arrived. First order of business: pray for good weather. This is no time to curse global warming.

"We've had snowstorms and ice storms and the city's shut down and everybody's gone and we're still out here--standing around the tree, singing carols," said Bisch, who has overseen preparations for the tree lighting and peace pageant since 1983.

For the big tree, there are 5,000 feet of lights--75,000 bulbs--and large evergreen bough and poinsettia flower ornaments. Each little tree gets 173 feet of lights, or 225 bulbs, and 50 ornaments made by the various states and territories. Warming tents need to be erected, bleachers built, a stage constructed and miles of cable laid.

"I love it even when things aren't going right like, electrical failures, and we have to check circuits," said John W. Glodeck, of New Carrollton, a nine-year veteran of light stringing and ornament hanging. Yesterday, he was dressed for outdoor work: overalls, boots, a coat and an "I Believe in Santa Claus" red and white flannel hat.

Believe in Santa Claus? "Hell, yes," said Glodeck, who keeps busy the rest of the year building and tearing down trade show sets. "I'm Santa to my grandchildren."

The tradition of lighting a National Christmas Tree began in 1913, when, according to historical accounts, 20,000 people crowded the East Plaza of the Capitol on Christmas Eve to hear the Marine Band and a chorus of 1,000. President Woodrow Wilson and Vice President Thomas Marshall lit the tree, and there was a presentation of the Nativity.

A similar ceremony was held the next year, but World War I forced cancellation of the event in 1915. It resumed in 1918 as a celebration of the end of war.

In 1923, the first year at the Ellipse, President Calvin Coolidge pushed the switch box button to light a 60-foot cut fir.

Since then, the annual tree-lighting tradition has continued--embellished by adding live radio, then television coverage and special entertainment--with some notable exceptions.

From 1942 through 1944, the tree wasn't lit because Washington was under a World War II blackout. In 1963, the lights weren't turned on until Dec. 22, when President Lyndon B. Johnson concluded a 30-day period of mourning after the assassination of President John F. Kennedy.

President Jimmy Carter lit only the star atop the tree in 1979 and 1980 because of the Iranian hostage crisis.

In 1980, it was fully lit for just 417 seconds, one second for each day the hostages had been in captivity. When they were released on Jan. 20, 1981, President Ronald Reagan had the tree quickly redecorated and lit briefly that day.

The ceremony has been known as "The Christmas Pageant of Peace" since 1954, when the arrangement of smaller trees, including one for the District, was added.

The live tree, whose multicolor lights will turn all white at midnight Dec. 31 to mark the new millennium, has become as fabled as Washington's stone monuments.

"I was explaining to Rachel that this is America's tree," Douglas Boag, of Ashburn, said yesterday as he watched preparations with his 6-year-old daughter and 2-year-old son, Bryan. "I think this is a wonderful way to show the United States focused on the capital."

CAPTION: Hargrove Inc. employee Judy Young holds ornament for the Maryland tree, one of many on the Ellipse encircling the National Christmas Tree, which lights up tonight.