Tens of thousands of Iowans spent part of the weekend signing and addressing Christmas cards and today is expected to be -the- busiest day of the year for the post office. Richard Watkins, spokesman for the U.S. Postal Service’s Hawkeye District, says this could be the busiest day for the postal service, not just for the year, but ever.Watkins says Iowa’s largest mail processing center in Des Moines handles 400-thousand cards, letters and packages on a normal day. Today’s volume is expected to hit one-point-one million pieces of mail, more than double and nearly triple normal. Nationwide, the post office is expecting 20-billion pieces of mail to be processed between Thanksgiving and Christmas. To speed things along, Watkins suggests using the recipient’s complete address, typed or printed, and be sure to include your return address on the envelope. As for the ZIP code, he says, don’t ever guess at it, as he says the volume of mail will make it tougher to find codes.He says using ZIP-plus-four, that four-digit number behind the ZIP code, is also a big help in getting mail to its destination more quickly. Post offices will be swamped today and customers can expect lines, though Watkins says people who drop by during “off” hours might get speedier service, like between 10 and 11 AM or between 2 and 3 P.M. His other advice? Go early, as he says the earlier you go in the week, the better. People who need a ZIP code or more information can surf to: “www.usps.com” to use the online ZIP code locator or call 800-ASK-U-S-P-S.