Centuries ago, according to a popular theory, some Asian astrologers became concerned that the lunar new year began inauspiciously in winter--a time when the hours of darkness exceed those of light.

To ensure better fortune for the people of the area that is now Cambodia, Laos, Thailand, Sri Lanka and Burma, the astrologers decided to switch the start of their year to April, when spring's longer days offer "excellent prospects of brightness and prosperity," in the words of a Laotian expert.

For these Southeast Asians, including those who've immigrated to the Washington area, the year 2525 began yesterday, the high point of several weeks of observances.The new year, the major holiday in these countries, has taken on added significance for the 2,500 Cambodians and 3,000 Laotians here, most of them refugees of war. It's become a time to get together with other refugee families scattered throughout the East.

Thongsoun Pathoumxad, a white-haired, 68-year-old Laotian woman who arrived here nearly two years ago and lives with a daughter in Falls Church, said she looked forward to Pimay Lao, the Laotian name for the holiday, "because it's time to meet friends who live in the neighboring states" and because it is a time for "happiness, to ask elders for forgiveness and to renew respect and devotion to the Buddhist religion."

Yet she, like many Laotian and Cambodian residents, said the traditional joy of the holiday is tinged with sadness because her family has been scattered. Two of her children are with her in Falls Church, but one daughter is in Paris and another is in Vientiane, the Laotian capital. "This is a terrible experience for my family as the result of the Communist takeover," she said.

Keo Vilaysack, a Laotian refugee who arrived here recently with his wife and two children and now lives in Northwest Washington, said he expects the celebration in America to be "quite different," but added that he and his family will "try to do whatever we can to preserve our beautiful culture, no matter where we live."

Many Cambodian refugees, however, have mixed feelings about the holiday and retreat into school work or jobs to avoid socializing.

Vissoth San, a 21-year-old student living alone in Northwest Washington, said he feels lonely, depressed and homesick because his large family remains in Cambodia. "The last time I had a New Year celebration was six years ago," he said. "I am looking forward to this holiday. I have a lot to catch up on."

Phinoun Lim, a 17-year-old Cambodian who lives in Fairfax County and is in 10th grade at Mount Vernon High School, said his memories of the holiday in his homeland are vague. "I was too young to participate in any of the dancing or games," Phinoun said, "and, as far as I can recall, there was always the shadow of war." But Phinoun, who hopes to become an engineer, said he's optimistic about the future. "Three years ago, I never dreamed that I would be in America," he said. "Everything is so great now--especially with the New Year coming."

For the Cambodian holiday, called Chaul Chnam Tmey ("the entrance of the New Year"), food is taken to the Buddhist monks and worshippers wear their best outfits to visit the temples. It is customary for each family to set up in their homes a white-clothed table bearing fruit baskets, flowers, traditional cakes and a large bowl of scented water. Incense sticks and pure beeswax candles, also part of the ceremonial table, are lighted at the exact moment the Chaul Chnam begins.

In Laotian custom, each house must be put in order on the last day of the old year to ward off disaster and banish the "evil genie." Candles and incense sticks are lighted, and people pray that the New Year will bring them health, wealth and happiness.

In addition to New Year parties and religious ceremonies that took place last week, three other observances are scheduled during the next two weeks:

* Lao New Year celebration featuring arts and craft displays, a traditional Baci ceremony, classical dance performances and traditional food. Saturday, from 11 a.m. to 2 p.m. at the Renwick Gallery of the National Museum of American Art, 17th and Pennsylvania NW, Washington.

* Cambodian New Year celebration. Sunday, 11 a.m. to 2 p.m., at the Renwick Gallery.

* Lao religious ceremony. April 25, 11 a.m. to 1 p.m., at the monks' residence, 5248 Clifton St., Alexandria.