30th day of the month
30 days
「三十日」 has another reading with different meanings.
last day of the month
九月は三十日で終わる。
September ends on the 30th.
三十日の夜に送別会が予定されている。
A farewell party is scheduled for the evening of the 30th.
月末の三十日は経理部が忙しくなる。
The accounting department gets busy on the 30th at month-end.