Although commonly referred to as 'snail-mail', Japan's postal system is highly efficient, easy to use, and ordinary mail is not that expensive. Post offices can be recognized by a white and red T with a bar across the top.
Red mailboxes are for regular mail and blue ones are for special delivery.
Other local post offices are open from 9 to 5 on weekdays and 9 to 1 on Saturday. There are some main post offices in big cities that are open 24 hours. If you are planning on having mail sent to you to a post office in Japan, it's a good idea to have it sent to one of the larger main post offices. Mail is usually only held for 30 days before it is returned to the sender, and smaller post offices may have trouble dealing with it. It is also possible to have your mail held at your embassy or at large hotels that are used to catering to foreigners.