As I mentioned in the New Years Apple ad post, nengajo feature an animal based on the Chinese zodiac. When you buy your pre-printed nengajo (just add addressee!) you will find the requisite beastie somewhere on the card. For 2007 the otherwise fearsome wild boar has been rendered as a cute boar piglet, evidenced by its stripey back, perhaps in an attempt to stop people thinking of the animal as food. Note too that Japan is referred to not as Nihon, but the rather more formal Nippon (same kanji: 日本)
The characters 年賀 (nenga) indicate that the card is indeed a New Years card and should not be delivered immediately (very socially awkward) but on January 1. If you receive a nengajo from someone but have not yet sent them one you can join the mass of people sending catch-up nengajo in the week following January 1.
The date is given in Japanese era format 平成十九年 - the 19th Year of the Heisei era - and Western format (西暦 , seireki) so don't think you can recycle any unused nengajo for use next year...
At the bottom is another wild boar, accompanied by a watermelon and the word お年玉 otoshidama. Otoshidama usually refers to the pocket money children receive from doting grandparents in the New Year but it also refers to the nengajo lottery drawn on January 14. Post Office issued nengajo have a lottery number, so holding onto them is not only a good way to boast about how popular you are, you may also win a fridge.
The question of why China uses Year of the Pig but Japan has Year of the Boar piqued my curiosity, so language geek that I am I had a look at the kanji used for each animal. Japan uses 豚 (buta）for the domesticated porcine and 猪 (inoshishi) for the wild relative which tastes great in nabe. According to Wikipedia, in China the character 豬 is used for the zodiac animal referred to as Pig , but the kanji looks a lot more like the Japanese character for Wild Boar....The two kanji, though similar, are not identical - they share a common component (者) but the left-hand side radical in inoshishi 猪 is kemonohen 獣編 and in the Chinese character it is inokohen 豕編.
So why the differentiation? I'm guessing that the Chinese and Japanese have always been talking about the same animal - scientific name Sus scrofa - and that somewhere along the line an English speaker got the two mixed up. Etymologists feel free to wade in.
By the way, do you know why the Pig/Wild Boar is the twelfth and final animal in the zodiac? According to the tame and most commonly known version the Pig/Boar slept in and so was late for the ceremony in which the Jade Emperor awarded the animals with zodiac rankings. In reality there was a very good reason the Pig/Boar was so tired - apparently, and this is no joke - he had spent the whole night porking.