Post box in Japan is red color and it was started to be placed since 1871. There were only 500 post boxes in 1875 but now are more than 190000 nationwide.
Mails are collected on designated times not only weekdays and Saturday but mostly on Sunday and public holidays. On the other hand, ordinary mails are also delivered on Saturday.
Modern post box has two apertures or inlet to put the mails inside the post box. It used to be local and other destinations but now changed to ordinary or express.
By the way, stamps for post card sending to Europe is just JPY 70, much cheaper than in Czech Republic. Stamps can be also bought at convenience stores which are open 24 hours a day.