Hundreds of post offices will stay open late Monday to accommodate last-minute tax filers. And in the New England area, they'll be open even later than that.

Throughout most of the country, income-tax returns must be postmarked by midnight April 16, and the Postal Service expects as many as 40 million tax forms to be mailed on that day.

Many larger post offices will remain open until midnight, and in some cities a party atmosphere prevails, with local bands playing and sponsors offering refreshments during the evening.

But some taxpayers in the Northeast are getting a day of grace this year. Those who file their returns with the IRS service center in Andover, Mass., have an extra day to get their returns in.

That's because Monday is Patriots Day, a state holiday in Massachusetts. As a result, taxpayers who file with Andover get a reprieve of one more day. Their returns must be postmarked or received by e-mail by midnight on Tuesday.

The Postal Service offered the following advice to tax-filers:

—Use first-class postage. The Internal Revenue Service accepts the postmark as proof that a return has been mailed on time.

—Use pre-addressed labels where possible. Take special care to ensure that handwritten envelopes are legible.

—Include a return address.

—Affix sufficient postage. One ounce is 34 cents; each additional ounce up to 13 ounces costs an additional 21 cents. Tax agencies do not accept postage-due mail and forms bearing insufficient postage will be returned, meaning taxpayers may be penalized for filing late tax returns.