It's a tradition for Japanese people to write New Year's cards during December. It's important to write "nenga" below the stamp or use prepaid New Year's postcards issued by Japan Post Network so that postal workers can distinguish New Year cards from regular mail.
Prepaid New Year's postcards with lottery numbers are the kind most commonly used by Japanese people. The lottery's winning numbers are picked in mid or late January. The prizes aren't money but are various goods, such as electronics, stamps, and so on. Those prepaid postcards are basically plain, so people print photos or images, draw pictures, and write messages on their own.
Another important part of the etiquette is not to send a New Year's card to a family in mourning. You might receive postcards from people who are in mourning from mid November, notifying you that they don't take and send New Year's greeting cards.
(as of 2010)