Refers specifically to the final calendar day of a given month. Common in billing, contracts, and scheduling contexts.
Please send the invoice by the last day of this month.
契約は来月の末日で終了します。
The contract ends on the last day of next month.
Please pay this month's invoice by the last day of the month.
This contract will end as of the last day of this month.
Salary is paid on the last day of every month.