What was going on when I was born?

Henry Willets Underhill regrets that he ever agreed to sell 81 acres of a 200-acre Jericho, L.I., farm that his family had held since 1793. Mr. Underhill was persuaded that the scenic beauty of the land would be preserved when it was developed. But the plans now call for leveling a hill and filling in seven ponds to accommodate 220 single-family houses.

President Elias Sarkis met with the Ambassadors of the United States and the Soviet Union today and reportedly appealed for their intervention to stave off a feared Israeli invasion. Contributing to the heightening of tension here were reports of Israeli troops movements near the border, continued Israeli threats of reprisals for the slaying last week of an Israeli diplomat in Paris, a reported regrouping of Syrian forces in Lebanon, and reports that the Palestine Liberation Organization has put its guerrilla forces on maximum alert. ''We are doing our best to urge maximum restraint on all sides,'' the United States Ambassador, Robert Dillon, said after his meeting this morning with President Sarkis. Mr. Dillon said Washington was in touch with the Israeli Government and the other governments in the area over the situation in Lebanon. 'Very Serious,' Russian Says The Soviet Ambassador, Aleksandr A. Soldatov, said after his meeting with President Sarkis that the situation in southern Lebanon was ''very serious.''

We are living in a most peculiar musical age. Musical life is booming, audiences are growing, seasons are expanding, conservatories are turning out virtuosos. In New York, well over a hundred concerts are given every week. There is an extraordinary bustle and whirl in the music world and its accompanying business. But in the midst of all that activity, there is a certain stillness, an immovable center. For our musical life is based upon repetition. In recent weeks, for example, the New York Philharmonic and the Metropolitan Opera have announced their programs for the coming season. At the Philharmonic, there are, of course, some unusual offerings. A concert performance of Janacek's ''From the House of the Dead'' is planned as are programs devoted to Shostakovich and to the Polish modern, Witold Lutoslawski - all reflecting growing interest in Slavic and Eastern European composers. There is also a ''retrospective'' of six compositions by Schoenberg planned, concentrating on his earliest music.

BRIDGEPORT AS most of Connecticut's hundreds of thousands of Protestants and Roman Catholics gather for joyous Easter services and festive dinners today, an often somber Holy Week is beginning for the 300 parishioners of St. Dimitrius Rumanian Orthodox Church here. Tonight, the Rev. Dimitrie Tatulescu will hang the purple drapes, symbol of the mourning to come, in the church. In many homes, families will eat lentil soup or tarapaschi, the traditional corn porridge, in keeping with the Lenten prohibition on meat and dairy products. Some women will be buying lamb, walnuts and feta cheese for the Easter feasts to follow. And congregants from as far as Hartford and New York City, and perhaps a relative or two from the old country - all bound by their roots in the Macedonian region of southern Rumania and northern Greece - will be planning the journey to the Clinton Street church.

THE brand managers at XYZ multibillion-dollar packaged-goods companya reconvinced that the company's new product is the best thing since sliced bread. The hotshots on Madison Avenue think they've created an unforgettable advertising campaign. Everybody is keen on sending his particular banner up the proverbial flagpole. But will anyone salute? To answer that all-important question, companies engage in an often expensive, lengthy and frequently inconclusive exercise known as test marketing, in which they try out their new product or marketing program in a handful of towns that mirror, as nearly as possible, the demographics and buying patterns of the nation. It is an effort to determine not merely how a product or marketing strategy will play in Peoria - itself a popular test-market town - but, based on Peoria, how it will be received nationwide. Because a national introduction may involve outlays of millions of dollars, test markets are a safety net under mass-marketers' slender highwire.

NEWARK FOR Brenda Grier, one of the seven announced candidates for Mayor of the state's largest city, last week was one of those when you win some and you lose some. Mrs. Grier lost some when she was informed by the City Clerk's office that her nominating petitions for getting on the May 11 ballot were being rejected because about 700 of the required 1,076 signatures were not valid. She won some when she was informed that an Administrative Law judge had ruled that her ouster from the Newark Board of Education by Mayor Kenneth A. Gibson seven months ago was invalid and had ordered her reinstated immediately. The nominating petitions of two of the other mayoral candidates were also invalidated. Former Municipal Court Judge Robert Brennan was told he was short by 102 valid signatures -among the signatures invalidated was his own - and Laura Garza, the Socialist Workers Party candidate, said she was about 100 valid signatures shy of the required 1 percent of the city's registered voters.

NERVOUS is the word for the time-share industry. This is the i ndustry that, for an upfront charge of several thousand dollars and a nnual maintenance fees far lower than the rates at comparable r esort facilities, gives buyers assured weeks of vacation time at a r esort. Time-share apostles are nervous because they know that questionable marketing and sales practices by even a few of the development organizations will damage sales and bring on a regulatory backlash. In their concern, leaders in the field have developed a model time-sharing act, still undergoing revision and tightening, that has formed the basis of protections adopted in several states.

While President Reagan took it easy on a ''working holiday'' in Jamaica and Barbados last week, American diplomats and Congressmen in El Salvador tried to mold a moderate government out of an election that favored the extreme right. At the Administration's urging, an eight-member Congressional delegation led by the House majority leader, James C. Wright Jr., flew to San Salvador.

President Reagan, in a live fiveminute radio broadcast to the American people, today angrily denounced critics of his proposed cutbacks in the student loan program. Speaking into a microphone from the study of a hilltop mansion overlooking the Caribbean, Mr. Reagan asserted that students had been deliberately misled to believe that the Government was ''snatching away'' their loans and that they had been ''incited to stage protest demonstrations against what has been called Draconian cuts in student aid.'' Mr. Reagan acknowledged that the amount of Federal money to help students repay their guaranteed loans would drop to $2.4 billion in the fiscal year 1983 from $2.7 billion in 1982 under his proposals. But he said the total volume of loans being made available by private lenders was reaching a record.

ON-THAMES, England, April 10 - In the Marks and Spencers department store here today, Tony and Olive Williams paused over a rack of women's wool blazers, quickly agreeing that 39.50 was too steep a price. Then the subject of the Falkland Islands came up and the family unanimity vanished. ''I don't think it's worth losing any lives over,'' Mr. Williams, a lanky local government official in his early 40's, said. His wife, a secretary, immediately interrupted. ''At the same time, I don't think anyone can come in and take over part of another country. It's simply not right.''

Here Comes the Taxman... As debate about the third stage of the Administration's tax cut swirls in Washington, the costs and benefits of the first part of the program should start showing up this week as completed income tax forms pour into the I.R.S. On the positive side, the recession and last October's 5 percent cut in personal taxes have slowed the growth in individual tax receipts. This year, revenues are estimated above $351 billion, up 8 percent from 1981. By contrast, last year's receipts were up 13 percent from 1980. And the Treasury reckons that because of higher-than-anticipated estimated payments, refunds should rise 11 percent to more than $53 billion and average refunds are expected to exceed $700. But the boost those funds could give to retail sales and the economy is not likely to occur. With unemployment at 9 percent and interest rates high, much of that money is likely to be saved. The real boost to incomes - and to spending - does not come until July, when the second stage of the tax cut takes effect. ...There Goes the Deficit With the extra pressure from those refunds, government borrowings, which have been running at last year's pace thus far, are expected to rise sharply, putting further pressure on interest rates.

''Italian Art Now: An American Perspective,'' at the Guggenheim Museum, is a remarkable phenomenon. Where anthologies of recent work from other European nations at the Guggenheim and elsewhere have enjoyed at most a success of politeness, this one includes at least two artists - Sandro Chia and Enzo Cucchi - for whom dealers and collectors all over the world are standing in line. Others in the show may well be in the same position before long. Italian art is in, that is to say, in a way that has not happened for many, many years. For that matter it has been difficult ever since the death of Giandomenico Tiepolo in 1804 for an Italian painter to make a lasting international name for himself. There were good Italian painters in the 19th century, but no one is going to claim that Fattori and Segantini were prized by foreigners as the personification of Italy in the way that was true of Verdi in the opera house and of Manzoni in literature. Before World War I the Italian Futurists were very hot on the international scene - so much so in fact that they seemed to scorch the very air as they passed. But there was in many of their beliefs a proto-Fascist element that has not worn well. We really don't want to think of Italy in terms of people who thought that it was in making war that man found his greatest fulfillment.