The mobile portion of the late Alexander Calder's "Mountains and Clouds," which was to have been hoisted up to the ceiling of the Hart Senate Office Building atrium by Christmas, is still awaiting its very certain fate. It sits -- a large, sprawling sculpture -- beached on the shoals of the Hart building atrium floor, surrounded by a plywood fence.

That the aluminum "Clouds" have not yet found their way up to the sunnier recesses of the atrium ceiling does not especially bother the staff at the Architect of the Capitol's office, who concern themselves with the design and artistic content of congressional buildings.

"The important thing is that it be done correctly, not on January 15 or tomorrow or what have you," said Elliot Carroll, executive assistant to the Architect of the Capitol.

The sculpture, which was commissioned in 1975 specifically for the large airy interior of the Hart building, is composed of two pieces. The first, a stabile, will be installed on the atrium floor; it now rests completed at Segre Iron Works in Waterbury, Conn. The second, a mobile that was fabricated by the New York firm of Crystallizations Systems Inc., will be hung from the atrium ceiling.

"It was felt very important that the integration of art and architecture be expressed in that building . . . Calder was selected out of five others," said Carroll. Calder was refining the maquette for the sculpture on the afternoon of his death, Nov. 10, 1976.

Funds for the sculptures were raised by the Capitol Art Foundation, founded by former senator Nicholas Brady, who said he "regretted the fact that the Senate had deleted funds for the sculpture from the Hart building appropriation." Brady established the foundation "for the purpose of placing art in the Capitol and surrounding buildings," said Ede Holiday, a member of the foundation's board of trustees.

The foundation will not comment on the cost of the sculpture and its installation. "The money was raised quietly and privately," said Holiday.

Meanwhile, Hart building workers have to accommodate the enormous mobile stretching across the atrium floor.

"I've had to change the way that I would ordinarily come in the building," said Paul J. Allen, Sen. Christopher Dodd's (D-Conn.) press secretary. "It's rather disappointing," added Allen, who said that he had hoped "Mountains and Clouds" would be as impressive as the Calder that hangs in the atrium of the National Gallery of Art's East Building. "Now," griped Allen, "it's interesting mainly because it sits there." Host of 'Hamlets'

As part of the Library of Congress' "Many Ways: Shakespeare on Film and Television" series that began last week, "Hamlet Week" begins today with a screening of Laurence Olivier's 1948 "Hamlet." Tomorrow, the library will screen the 1920 version of "Hamlet" that offers a vision of Hamlet as a man who is actually a woman.

It gets better. "Crowns and Clowns: Lancastrian History Tetralogy" week begins Feb. 3, featuring television and film productions of "Richard II," "Henry IV" and "Henry V." The final coup de grace is March 17, with "Macbeth Week."

A variety of feature-length films dramatizing Shakespearean works will be screened between these special feature weeks.

Showings are free, in the Mary Pickford Theatre on the third floor of the library's Madison building. Reservations can be made for all screenings by calling 287-5677. Notes

The Washington, D.C., Chapter of the Artists Equity Association Inc. is sponsoring a free public symposium, "A Panel of Four: The Role of the Art Consultant and Artist in Today's Marketplace," from 2 p.m. to 5 p.m. Jan. 26, in the National Museum of American Art's lecture hall, Eighth and G streets NW. Reservations are required and can be made at 598-2424.