The beginning of the month; the first days of a month.
家賃は月初めに銀行口座から引き落とされる。
The rent is withdrawn from the bank account at the beginning of the month.
月初めは請求書の処理で忙しい。
I'm busy processing invoices at the start of the month.
給料日は月初めとは限らない会社もある。
Some companies don't pay salaries at the beginning of the month.