The middle ten days of a month (the 11th through the 20th), contrasted with the beginning (上旬) and the end (下旬).
今月の中旬に旅行に行く予定だ。
I'm planning to travel in the middle of this month.
期末レポートは来月中旬までに提出してください。
Please submit the final report by the middle of next month.
予報では今月中旬は暖かくなるらしい。
The forecast says it will get warmer around the middle of this month.