The Japan Mint instituted the 'Sundays' series of badges in 1948. Actually, according to the histories of the Mint that I have read, the first badge was created as a trial of some sorts and only available in small quantities. They proved to be a popular item, so the Mint has been making them every year. The first three (1948, 1949, 1950) medals are larger table medals, but from 1951 the medals became badges. These badges had no ribbon and are complete as shown below. The earlier medals and badges are not often encountered, but since they were popular, they are probably still gathering dust on shelves across Japan.
I have seen a few larger medals as well as badges of some of these, so it may be possible that both were minted for each year.
Most show the national holidays of Japan, too, and the animals represent the creatures in the Chinese zodiac.