Today is to taxes what Christmas Eve is to shopping, the last opportunity to pick up tax loopholes and deductions. Suddenly the anxious "haves" want to have not, or at least halve what the tax man takes.

So all around town people are winding up a busy week of stuffing gifts of checks and stock certificates into mailboxes and leaving clothes, furniture and even cars at Goodwill and the Salvation Army.

And hounding their accountants and stockbrokers until all hours.

"We have 45 tax attorneys and all of them are busy today," said Mortimer Caplin, a former commissioner of the Internal Revenue Service and a member of Caplin & Drysdale, a local tax firm.

Yesterday and today, said Caplin, "are like the Iron Curtain. Either you get in under it for this year, or you're out in the cold for '83." A tax deduction this year, said Caplin, is more important than next, when the tax rate is now legislated to fall by 10 percent.

Business is booming at several area charities and nonprofit groups, the traditional year-end beneficiaries. At Plan America, formerly the National Foundation for Philanthrophy, director Will Rose reported a "flurry of calls asking, 'If my check is postmarked on Dec. 31, but doesn't get there till January, is it still deductible?' " (The answer he gives: yes.)

"Come by after 3:30" today, said the Salvation Army's John Morris, "and you won't believe it. Last year we had three people out front handing out receipts and three more in the warehouse." Already this week Morris has received enough used cars to open up a lot. "We had three come in yesterday, and a couple this morning. Last year one guy brought one in a tow truck."

Charities aren't the only beneficiaries of the scramble to reduce taxable income and thereby cut taxes due. From stock trading at K Street brokerages, to computer sales in Silver Spring, to the rush to open tax-deferred Keogh accounts at local savings and loans, the annual tax sweepstakes yields bonanzas.

Arena Stage, which relies on contributions for 25 percent of its operating budget, logs a "sizable" increase in contributions this time of year, according to assistant director of development Tom DiGiovanni. "We show an increase in cash, and stocks of all kinds, everything from Sears Roebuck to General Foods."

Donating stock instead of cash can be lucrative, brokers say. Someone whose initial $1,000 stock purchase has appreciated to $5,000 can donate the entire $5,000 in stock, take a tax writeoff for that amount, and not pay the capital gains tax on the $4,000 profit, they point out. Charities, local brokers say, almost always cash the stock in immediately.

"It's very busy, the phones are ringing constantly," said a downtown stockbroker. "A lot of people are off this week and they're finally getting around to their taxes. We see a lot of people rushing into last-minute tax shelters."

And "tax selling," or the unloading of unprofitable stocks at the end of the year to pick up losses and thereby more deductions to offset other stock gains, is also up. The seller can replace his loss with a similar stock or bond the same day, and bargain hunters pick up the jettisoned stocks at a good price, brokers said.

Local computer companies report a surge in post-Christmas purchases by customers who say they must have their computers in place by today. Jim Swain of Computers, Etc. in Silver Spring, for example, echoed other stores in saying he had been "swamped" all week.

Tax laws require that a computer for business use be in place and in use no later than midnight tonight to take advantage of some special benefits in the '82 tax year. "I just had a guy in here who said he needed to take it home," Swain said yesterday.

Sometimes it's not so much what you do before year's end as what you don't. This year, untold marriages have been delayed to avoid filing joint tax returns; divorce settlements, too, have been postponed to keep costly property transfers off the '82 returns.

Elizabeth Guhring, a longtime Washington divorce attorney, offered what she called a "perfect example": a prominent Washington couple whose divorce has been deliberately delayed until the opening of court Monday morning, although the divorce agreement has been signed for two months. "This way they file a joint return, and this is of great benefit to them."

"I have other clients," said Guhring, "who are waiting until Saturday to be married so they can file separate tax forms this year" and avoid the so-called marriage penalty, the higher tax rate that often results when two high incomes are joined in holy matrimony.

If all this makes you long for a cigarette, be aware that even that may be better today than tomorrow. After Jan. 1, the excise tax manufacturers pay on cigarettes will rise from 8 to 16 percent, and thousands of cigarette retailers have spent the week stocking up to beat the clock.

"Virtually every retailer that wants to make a windfall profit is out buying cigarettes," said Joe Yoshbi, comptroller at Capital Cigar, one of the area's largest tobacco wholesalers. He said Capital has had every delivery truck it could spare on the road for days.

Rodman's Discount Drugs Inc., for example, has purchased 3,000 cartons and stacked them in a locked office. It plans to pass the savings along to customers, said assistant manager Gary Neuman. But many retailers, said Capital's Yoshbi, will hold their cache for a week or so, sell at the new, high price, and pocket the difference.

"The retailers see an opportunity to buy a carton 44 cents cheaper today than tomorrow. Today is it. Tomorrow, the party's over."