If you want to be totally professional.

And show you are in the driver's seat,You can write: 

"Thank you for the rent. As stipulated in our lease agreement, the rent is due on the first of each month. Appreciate your cooperation going forward, thanks!"

其他廢話少說. （我們的合同5號之前收到都OK， 你去讀一下你怎麽

和她簽合同的， 我寫的英文是假設你合同裏有每月1號）