The Senate finished decorating its own version of the national Christmas tree yesterday by adding all manner of goodies -- including a provision putting off the planned nine-digit zip code -- to its omnibus spending bill.

After acting on 155 amendments during three days of debate, the Senate, by a vote of 56 to 22, finally approved the spending measure to finance most government operations through the rest of fiscal 1981. The measure now goes to conference with the House.

The spending package was the last major obstacle to adjournment of the 96th Congress, and Senate Majority Leader Robert C. Bryd (D-W.Va.) said last night that Congress probably will be able to adjourn tomorrow or Monday.

Earlier, as the Senate appeared hopelessly bogged down in amendments, the pre-Yule merriment finally drew a tongue-lashing from Byrd, who warned the Senate it was jeopardizing any hopes of quick adjournment of Congress by "making a Christmas tree" out of the theoretically routine spending measure.

The Senate, he declared, was engaged in a "needless, profane waste of time" as it added dozens of amendments to the measure, most of which will be brushed aside in a House-Senate conference.

"What are we doing?" Byrd exclaimed angrily. "Playing to the headlines back home!" he responded to his own question.

What triggered Byrd's outburst was more than an hour's debate, followed by a time-consuming roll call resulting in a 90-to-0 vote, on barring the Postal Servicce from spending any of its federal subsidy until at least June 1 to carry out its planned expansion of the zip code from five to nine digits.

As modified under pressure from senators who support the nine-digit plan, the zip code amendment, advanced by Sen. David Durenberger (R-Minn.), would permit the Postal Service to continue buying equipment and making plans for the conversion. It just couldn't use any of its subsidy to put the program into use, which it had planned to do sometime early next year.

"My concern is that we are spending at least $1 billion, and probably more, to buy . . . a Model T for an Indy 500 race," complained Durenberger. m

Then he painted a dismal picture of how the average citizen would have to cope with the new system. "Consider the horror of poring over almost 20 million zip codes listed in a 30,000-page directory -- a directory that is more than 40 times the size of the District of Columbia white pages, weighing in excess of 80 pounds, to send one letter," said Durenberger.

However, Sen. John Glenn (D-Ohio) said the cost would be more like $40 million to $80 million and would result in cheaper and better postal service, with little inconvenience to the public and net savings of $597 million by 1987. In the end, Glenn went along with Durenberger's proposal after it was modified to permit the Postal Service to keep moving on the plan.

Before tacking this and other ornaments onto the spending resolution, the Senate flinched at a rather more painful proposal to cut spending by an estimated $4.6 billion to keep the total from busting the $632.4 billion budget ceiling that Congress adopted two months ago.

The proposal by Sen. William L. Armstrong (R-Colo.) was rejected 42 to 45, with many Democratic senators who are up for reelection in 1982 joining most Republicans in almost passing the amendment -- an indication that similar retrenchment ideas may fare well in the Republican-controlled Senate next year.

With the failure of Armstrong's proposal, the spending resolution will fund many agencies of the government until the budget ceiling is reached. It does not say what happens next. Presumably either the Reagan administration or Congress will come up with spending cuts, or Congress will, as it has done before, simply raise its budget ceiling.

Before finishing, the Senate voted to continue to prohibit the Internal Revenue Service from issuing regulations affecting the tax-exempt status of private schools and agreed to continue permitting Medicaid abortions in cases of rape or incest as well as jeopardy to the life of the woman. The House version permits Medicaid abortions only when the woman's health is at stake.