What was going on when I was born?

President Reagan's chief tax-policy advisers and the members of Congress most closely involved in writing tax legislation agree that the House of Representatives will quite likely pass legislation this fall embracing the main principles of the President's plan to redesign the Federal income tax system. The measure, they are confident, would reduce tax rates for individuals and corporations, increase the personal exemption allowed taxpayers and their dependents, abolish many deductions and credits and, based on the best available estimates, raise neither more nor less revenue than the present tax law. Beyond that, there is no consensus among the more than a dozen key players interviewed last week on what will happen. Some people think the particular elements of the House bill - the rate structure and the specific deductions to be retained and repealed, for example - will closely resemble the President's proposals. Others think the bill will be vastly different from what Mr. Reagan recommended.

WELL, yes, there's this squabble about money for the rookies, agents insisting on more and club owners holding the line. But as the National Football League begins its 66th season this week, with the first training camps opening, most of the bigger headaches are still a year away. By this time next year, preparations will have begun for negotiations on the next series of contracts with ABC, NBC and CBS and for the next collective bargaining agreement with the players' union. Those two items, through which the 28 clubs will receive and pay out vast sums of money, will shape the football industry for years to come. Until then, the N.F.L. is expecting a season of serenity, give or take a few holdouts, with high expectations based on the United States Football League's yearlong hiatus, an influx of quality players from the draft and other leagues, a streamlined game and the emergence of several new powers among the clubs.

If the Landmarks Preservation Commission agrees, before long there will be a 19-story apartment tower squeezed behind the facades of a trio of old brownstones on East 79th Street just off Park Avenue. The brownstone fronts will remain, essentially as false fronts behind which the new tower will rise. If this unusual hybrid building goes ahead, however, it may not be the only such structure in New York: the commission has already given its nod to a much larger, but essentially similar, building, a 57-story tower designed to slip behind the facades of the landmark Rizzoli and Coty buildings on Fifth Avenue between 55th and 56th Streets. The two buildings were both planned by distinguished architectural firms -Conklin Rossant at 79th Street and Kohn Pederson Fox at Fifth Avenue. Though neither is ideal, both designs follow the current fashion of integrating distinct historical elements into a larger whole, and they are both among the more sophisticated essays in this eclectic style.

A diagram in some copies yesterday accompanying a description of President Reagan's surgery failed to show the full extent of the section of bowel removed. In the procedure, a right hemicolectomy, approximately two feet of bowel were removed, beginning with part of the transverse colon and continuing along the entire ascending colon, plus the cecum of the colon and part of the adjoining section of the small intestine called the ileum.

President Reagan's doctors said today that he was making a spectacular recovery from surgery and White House officials prepared for a long convalescence period in which he will carry a light work load. Mr. Reagan greeted White House aides a day after undergoing major abdominal surgery by telling them, ''I'm amazed at how good I feel,'' said Larry Speakes, the White House spokesman. Mr. Speakes said that Mr. Reagan's vital signs ''were good'' throughout the day. The President spent part of the day sitting up for about 45 minutes, reading a Western novel and watching television with his wife, Nancy, who arrived at the hospital at about 10 A.M. He also took a brief walk around his room, Mr. Speakes said.

''I'm amazed at how good I feel.'' - President Reagan, recovering from surgery. [A1:6.]