Every year, the inevitable April deadline arrives, bringing on last-minute scrambles for tax advice, late-evening dashes to the post office to mail tax returns, and an outpouring of gripes and musings about the quirks of taxes and taxpayers.

At midnight last night, this year's deadline occurred for about 4 million taxpayers in Maryland, Virginia and the District of Columbia who, along with 87 million other American taxpayers, are required to file federal income tax returns. And as the count-down neared its end, the pace quickened at crowded tax offices here.

"This is the most hectic day," said Paulette H. Sewell, who manages taxpayer services at the Internal Revenue Service's Washington-area office, 1201 E St. NW. By 4 p.m., her small staff of nearly 20 federal tax advisers had already helped 495 taxpayers fill out returns.

"Tiring," said one of Sewell's staff representatives, summing up in one word her views about the day's work. Then she posed a rhetorical question about taxpayers' quirks: "Where were they when we were sitting around doing nothing?"

For taxpayers waiting in straggling lines outisde the IRS offices, the day's delays were greeted with a mixture of philosophical good humor and harried annoyance.

"I usually don't file until the last day anyway, because whatever I get back (as a tax refund)-that's my vacation money," said Rylander L. Humble, 52, a night maintenance foreman for the Montgomery County school system who lives in Northwest Washington, as he waited near the end of a line outside the IRS. offices. "I don't mind (waiting in line). I'm patient."

At another IRS office, Mary McCan, a D.C. public school teacher, joined dozens of others who were seeking a reprieve from yesterday's deadline. "I came to get the extension form, and I think practically everybody else (in the line outside the office) did too. I have had broken right arm and I have not been able to write."

The IRS automatically grants a 60-day extension to any taxpayer who filed a request for a delay on Form 4868 by midnight last night, an IRS spokesman said. McCan put Form 4868 in her pocketbook.

James Greggs, who directs a non-profit cultural art scenter in Far Northeast Washington, had inched his way toward the front of the line outside one IRS office, but he was not happy. He expressed annoyance at the long line and the inconvenience of having to travel by bus and subway from his home in Far Northeast to a downtown IRS office to get tax advice. "It's terrible," he said. "It's extremely slow."

A 61-year-old retired post office clerk shrugged off the meandering lines and delays. "I'm like that every year. I just put it off until the last minute," he said, chuckling. "Just lazy, I guess."

Midnight last night was also the deadline for about 300,000 D.C. taxpayers and nearly 1.7 million Maryland taxpayers to file their city and state income tax returns. Virginia allows its more tha 2 million taxpayers until May 1 to submit their state income tax returns. But a Virginia Taxation Department spokeswoman said that nearly 1.6 million returns had already been filed and many more normally arrive at about the same time as the federal income tax deadline.

The federal income tax deadline, normally April 15, was advanced one day this year because April 15 fell on a Sunday.

Not only do Virginia residents have about two weeks more time to prepare their state income tax returns, but they also pay lower income taxes, according to government officials and private accountants.

John Potter a tax service manager for H&R Block, Inc. here, the biggest private tax assistance firm, took a few minutes off yesterday to calculate some regional variation. He found that a single taxpayer who earned $20,000 last year and claims the standard deduction on his tax return would pay $1,313 in Maryland income taxes, $1,275 in D.C. taxes and $764 in Virginia. A family of four with $30,000 in income and some typical mortgage interest and other itemized deductions, he computed, would pay $1,449 in the District, $1,394 in Maryland $964 in Virginia.

A D. C. government study, based on tax data for 1977, found similarly that income taxes ranged in the District of Columbia from $34 for a family of four with a $5,000 income to $3,405 for a four-member family with $50,000 in income. In Maryland, the study said, the taxes for similar families varied from $42 to $2,762. In Virginia, it reported, the range was from $26 to $2,012.

The Virginia Taxation Department spokeswoman also said that, according to 1976 tax data, average income taxes pin Northern Virginia were $526-less than the $567 average for the District and the $603 average for Maryland.

Whatever the tax bill, most statistics showed that yesterday was, indeed, the day of the annual income tax deluge.

George Conrad, a spokesman for the Post Office locally, said that an estimated 175,000 tax returns would flood the Washington area's postal system yesterday. As usual, Conrad noted, about 15 uniformed letter carriers were assigned to stand along Massachusetts Avenue and North Capitol Street near the main Post Office building here last night to collect tax returns from passing motorists, seeking to get their returns postmarked before midnight.

Across the nation, the IRS spokesman said, about 10 million returns were expected to be filed yesterday. CAPTION: Chart, shows D.C., Maryland and Virginia income taxes for a family of four; Picture, Postal worker Lionel Merchant picks up a tax form from a passing motorist at Washington's Main Post Office to help him beat the midnight deadline. By Joe Heiberger-The Washington Post