LOS ANGELES (CBS) — If you’re sending cards or gifts Monday, be sure to pack your patience for what is expected to be the busiest mailing day of the year for the United States Postal Service.

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Be sure to send items by the following dates if you expect them to arrive by Christmas:

Tuesday, December 20 – First-Class cards and letters

Wednesday, December 21 – Priority mail

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Thursday, December 22 – Express mail

If you’re sending gifts to a big city, Express mail can be delivered as late as Christmas Eve.

USPS officials also recommend using eco-friendly air-popped popcorn to stuff your boxes instead of plastic peanuts.

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For more shipping information from USPS, click here.