CHICAGO (CBS) – If you’re headed to the post office today, you could be greeted with some pretty long lines.

WBBM Newsradio’s Nancy Harty reports today is the busiest mailing day of the year.

The line at Chicago’s Main Post Office at 433 W. Harrison St. was full of people sending out holiday packages and letters, but was moving smoothly.

“We’ll probably do about 4 million letters mailed in the Chicago area alone,” said U.S. Postal Service spokesman Mark Reynolds.

LISTEN: WBBM Newsradio’s Nancy Harty Reports

He said the biggest growth the post office has seen is in packages.

One woman in line was waiting to send a box of winter clothes to her niece, who celebrates her first birthday just days after Christmas.

“This is actually pretty good. I thought it was going to be longer than this,” she said.

Another woman in line said she considers herself ahead of the game, having exchanged some gifts with out-of-state relatives over Thanksgiving.

“I expected a line coming in, but honestly it’s not too bad,” she said.

Reynolds said timing is key for avoiding crowds at the post office.

“The earlier in the day, the better; if you can avoid going at lunchtime, when the lines might be a little longer, or right after work,” he said.

Reynolds said postal workers will be making deliveries on Christmas Eve, and some even on Christmas Day.