The last third of a month: the period from the 21st to the last day of the month.
今月の下旬に出張があります。
I have a business trip scheduled for the end of this month.
締め切りは下旬までです。
The deadline is until the end of the month.
旅行は来月下旬に計画している。
The trip is planned for the end of next month.