Its not really the Japanese culture to send birthday cards and Christmas cards, however, they send lots of New year greeting post cards to their family, friends and co-workers. These post cards are called `Nengajo` （年賀状）.
In these post cards, they Japanese express their new year`s wishes to their relatives, friends and colleagues. There are really no restrictions in writing nengajo however, based on my experience, the Japanese still prefers cute designs with minimalist details.
In sending nengajo, timing is very important. The nengajo must arrive at it`s destination by January 1, so the post cards must be sent on or before December 25 in order to arrive on time.
So if you plan on sending your Japanese friends some Nengajo, it is recommended to prepare earlier. If your in Japan, you can opt to buy from the Post office, local book stores or even in convenience store. However, if you wish to send a unique and personalized card you can opt to make your own card. No matter what kind of Nengajo you send, it still the thought that counts.