Just curious why the number 20 seems to be special in terms of having a different pronunciation. Someone 'comes of age' at 20' .. what does that mean for a japanese person? Because you can say 'hatachi' when you are 20 years old.. and on a calender the 20th of month is 'hatsuka'. is there a reason for this?

Also for the calender all the dates use the chinese pronunciation 'shichi', 'ku' except for 4 which is 'yokka'.. is that to avoid the ominous 'shi' pronunciation since in Chinese it's the same pronunciation as 'death'? does that cross over into japanese?

Hi

I have no idea why Japanese uses the Japanese reading for the age 20 and 20th day of the month. I don't think it has to do with coming o age because that was raised after the Meiji restoration, it used to be closer to 14. This age is still celebrated in schools in Ehime prefecture, the only area of Japan that retained the tradition - fun fact for you.

Anyway, days of the month are all Japanese readings from 1-10 and 20. Chinese readings are used for all the other dates. The 9th day of the month is ここのか and the 7th is not しちにち but なのか.

ok.. it's just that the Genki 1 text said (in a footnote) that people say 'hatachi' when they turn 20 because they're 'coming of age'.

I looked at this -- http://gogen-allguide.com/ha/hatachi.html

it looks like the simplest answer is that はた (20) is an alteration of ふた (2). other explanations link it to 旗... that's discussed in more detail in the link.