The beginning of the month; often used in business and accounting (same meaning as "月初め").
月初に請求書をまとめて送ります。
I send invoices together at the beginning of the month.
給料は通常、月初に振り込まれることが多い。
Salaries are often paid at the beginning of the month.
プロジェクトのミーティングは月初に設定しましょう。
Let's schedule the project meeting at the start of the month.