By the end of this month; within this month (used to indicate a deadline or timing before the month ends).
レポートは今月中に提出してください。
Please submit the report by the end of this month.
彼は今月中に引っ越す予定だ。
He plans to move within this month.
今月中に申し込まないと、割引は受けられません。
If you don't apply by the end of this month, you won't get the discount.