1.
last day of the month
The final day of any month. Originally meant the thirtieth day, but now refers to the last day regardless of how many days the month has.
三十日に支払いをする。
I make payments on the last day of the month.
毎月の三十日に締め切りがある。
There is a deadline on the last day of every month.
二月の三十日は二十八日か二十九日になる。
The last day of February falls on the 28th or 29th.
ETYMOLOGY: 三十日 literally means the thirtieth day (三十 is an old counting word for thirty + 日, day). It has come to mean the last day of any month.
RELATED WORDS:
- 大晦日: New Year's Eve (literally 'great last day') — the last day of the year, December 31st
- 晦日: an alternate kanji spelling with the same meaning
USAGE: Less common in everyday speech than 月末 (end of the month), but 三十日 has a traditional feel and is still used, especially in set phrases and financial contexts. The derived word 大晦日 is extremely common.