Today is OO-MISOKA(大晦日): the last day of the year. OO means 'grand, large' and 'MISOKA' means the last day of the month. 'MISOKA' is consisted of 3 sounds 'MI'(three), 'SO'(ten) and 'KA'(day). 'MISOJI'(三十路) means 30 to 40 years old.
Why '30th day' had meant the last day, not 31th day? It is because the lunar calendar used in Japan before AD1873. The 31th day did not exist in those days.
Today most of Japanese eat a noodle 'TOSHI-KOSHI-SOBA'(beyond the year noodle). The long noodles represent the long life.