January 5, 1986

How can one explain the American fascination, to the point of exuberant idolatry, with Sherlock Holmes? The fascination finds its latest expression in ''Young Sherlock Holmes,'' the sumptuous period film produced by Steven Spielberg, an engaging trifle in which the great detective makes his imagined first bow as a schoolboy. The idolatry touches its seasonal peak on Friday, when the Baker Street Irregulars converge for their annual dinner at the Regency Hotel. That night, a hundred-odd more or less grown men will assemble to commemorate Holmes's 132d birthday. The evening always commences with what are known as canonical toasts - elaborate and sometimes labored tributes to Holmes, Dr. Watson and assorted bit players in the canon of Holmes's adventures, such as Irene Adler, the siren born in New Jersey who upstaged and outwitted the sleuth in ''A Scandal in Bohemia.'' This particular game has been afoot since the 1930's, when the Irregulars first began gathering to ponder the Sacred Writings at the call of Christopher Morley, Elmer Davis, Rex Stout, Edgar Smith, Vincent Starrett and a survivor among the founders, Julian Wolff, M.D., who still presides. American devotees have gone to remarkable lengths to establish that the detective and the good Dr. Watson were real people, and that the putative author, Sir Arthur Conan Doyle, was merely Watson's literary agent.

The Administration's concerns over Libya, long focused on that country's attempts to undermine neighboring pro-Western governments, have heightened in the last week with charges that Libya supported Palestinian terrorists who attacked the Rome and Vienna airports. If the charges are true - and the evidence is only circumstantial - it would be an ominous development, in the view of Administration officials. They say this is a new combination of three elements: the Abu Nidal group, one of the world's best-trained terrorist organizations, with a supply of young men willing to go on near-suicide missions, and backing from a Government with an aggressive record. ''It is getting more dangerous,'' said a State Department official who specializes in watching terrorism. More guerrillas seem to be ''willing to have their heads blown off,'' he said.

The Nest-Egg Effect The year may be over, but taxpayers have until April 15 to lower their 1985 tax bill by making contributions to their Individual Retirement Accounts. An estimated $230 billion will be invested in I.R.A.'s by that date, says Robert A. Schwartz, senior economist at Merrill Lynch Financial Economists. As they grow, the nest eggs affect more than the retirement plans of their owners - they change the outlook for major sectors of the economy. ''Some of the rally last year in the bond and stock markets can be attributed to a switch of I.R.A. funds from short-term certificates of deposit to stock and bond investments,'' said Mr. Schwartz. He estimates that some $85.6 billion of I.R.A.'s invested in C.D.'s will mature during this year's first quarter, down only slightly from levels reached in the final quarter of 1985.

The buildings that housed the old Newark News, structures that have remained unoccupied since the newspaper folded in 1972, are being renovated into condominium apartments. To be named Clinton Plaza, the project will occupy a square block in downtown Newark in an area bounded by Clinton, Beaver, Mulberry and Market Streets which is two blocks from the Pennsylvania Railroad Station and the PATH trains to Manhattan.

An artistic moment has passed. It is not that the phenomenon known as Neo-Expressionism is dead, or that the artists identified with it are no longer the subject of intense interest and debate. But the fire that was lighted by those European and American artists whose large-scale, usually painted work burst into prominence around 1980 with a flair and sweep that seemed to make everything possible, is now down to a glow. Together, it is unlikely that they will receive the same kind of attention again. The passing of a moment can be felt in many ways. Although with the most widely respected of the artists there may be no tailing off in prices or gallery attendance, there is far less sense of expectation and excitement, and far less interest in the artists as a whole. Furthermore, a new arts scene encompassing many approaches and styles has emerged in the East Village, and become itself a complex, international phenomenon. There are also new generations of artists, particularly in Italy and Germany, that the art establishment is now eager to look at, exhibit, write about and sell.