due day and deliquent day

Due day mostly will be 1st or the month. Deliquent on 5th day of the month.

The payment shall be count as late past due day (mail shall be stamped before due day).

Late charge apply for payment RECEIVED past deliquent day.

I think the few days difference is for the tenant to count the mail delivery time (mailed before due day) and some surprise delay (holiday, etc.) and can still reach the own by deliquent day.