Yesterday was the busiest day of the year at the Library of Congress, and once again the place was nutty, just as it's traditionally nutty on the day after Thanksgiving. It was like a white sale at Woodies. It was like National Airport the weekend before Christmas. It was like trying to find your sister at the New York City marathon.

They came with ratty notebooks and elegant briefcases. They brought bananas and term papers in their backpacks. A lot of them were college students, home for a long weekend, who had a paper due back at Brown or Birmingham Southern. But others were people who seemed desirous of gobbling something other than what they had gobbled at a table the day before.

By l0 a.m., it appeared that Tysons Corner had almost nothing on the library's main reading room -- only these were a different sort of comparison shopper. The studious and scholarly and merely browsing stood five deep at the copier machines, four deep at the computer catalogue terminals, which, of course, went down in the white heat of research. And all day long the tours. Possibly never in one room have so many people sought to whisper hoarsely.

Said a staff librarian named Tom Mann: "Oh, yes, somebody wants the history of frozen orange juice in Brazil, or TV dinners in Australia, or the number of casualties in Guadalcanal. They come stricken to you for help. It's fun. I mean, it keeps you hopping. Actually, I volunteered to work today. I took this job because I like to help people. And this is the day for helping, all right."

Mann, a likable sort with apparently vast patience, has his PhD in English. When he got out, in '75, the only jobs he could find were teaching frosh composition at community colleges. So for a time he worked as a private investigator for a detective agency in Louisiana. He quit to work at a library. Though he's worked at several good university libraries, he is now clearly employed at the Summa Theologica. As he talked yesterday, plaintive students kept coming up to ask him for help. A college woman wanted to know where she could find out about Chinese theater and its cultural exchanges with American theater. He put her on one of the reading room's 18 computer terminals and went to check on her later.

By 10:50 a.m., the full-to-capacity sign in the main reading room was up -- and still they came, spreading themselves and their looseleaves out at the fine old wooden desks under the fine old brass lamps. The main reading room hasn't changed much since 1897. Oh, they've taken out the gold spittoons and the gas lights with the clouded Victorian glass shades. But the desks still fan out in widening circles from the raised central desk. That central desk, under an impossibly vaulted ceiling, is like an altar. Behind it, the high priests of research (i.e. the librarians) dispense their grateful communions. Yesterday, a patron requested the "History of St. Michael's Church, Trenton." Somebody else filled out a slip for "132 Ways to Earn a Living Without Working."

There is something sensuous about all this enveloping pursuit of knowledge, all this learning, all these arcane and wonderful books, under one grand, mosaic dome. The main reading room is a 19th-century sanctum. In a world of chaos, it offers peace and learning. (Yesterday, it was scant on peace.) You walk in and are assaulted by a smell--no, an aroma: something woody and warm, something oddly atavistic, like a summer attic where childhood treasure lay waiting. Mixed in with this smell is the holy musk of studious seekers bent furiously to the pencil. It is the silent camaraderie of learning.

The Library of Congress is the largest library in the world, 80 million items, 532 miles of shelves, the papers of Sigmund Freud, the letters of George Washington and Branch Rickey, the largest collection of comic books known to man -- you name it, they've probably got it. Only the Lenin State Library in Moscow is any serious competition. The New York Public and the Harvard libraries are fine and huge, all right, but this one beggars imaginations. In the main reading room alone are 45,000 reference books, a card catalogue of 23 million cards. (Actually, the card catalogue spills out into another room.) The Shakespeare file covers 23 drawers -- and that's still not everything on him. As Librarian Daniel Boorstin says, the place is a multimedia encyclopedia.

"Let's face it," said a teacher named Greig DelaHoussaye who sat with three world geography books in his lap, "there aren't many places in the world where you can just walk in, take a seat and figure you're doing work in a room where some of the brainiest people who have ever lived have worked." DelaHoussaye used to be a managment consultant; now he tries to cram learning into seventh- and eighth-graders in Bethesda. He needed a change, and the challenge of it has been rewarding if rough. He said he intended to stay all day and try to get ahead on lessons and research. He had his lunch with him and figured he'd go outside on the lawn if it got warm.

Luise Woelflein, a senior at Brown University, was at desk 88. Around her lay killer tomes, one titled "History and Bibliography of Anatomic Illustration" by Ludwig Choulant. Woelflein was sort of mooning into studious infinity. She had on blue suspenders and argyle socks and looked preppy. She had a paper due for Prof. Benedict's history course, she said, and the real problem right now was figuring out what the heck her topic was. "I think something on the effect of humnanism on scientific thought in the early 16th century," she said. Woelflein lives in Georgetown. Somebody dropped her off at 8:35 a.m. and would be back for her at 5.

Elizabeth F. Stroup is director for general reference at the library, and yesterday, on her biggest day, she led a guest into a quiet corner of the Italian Renaissance foyer and pointed up at a mosaic. "On days when it gets too much, I just come out here and look at this," she said. Inscribed on a column was this: Knowledge is Power. "That's what we're about," she said.