Payment Dates 
Use this window to define the days each month when your customer usually pays the open invoices, or the days each month that your company pays its vendors.
To open the window, choose 
Business Partners 
Business Partner Master Data Payment Terms Payment Dates 
.
Structure
In the Payment Dates window, enter the payment dates of the business partner.
Example
When 10 and 20 are defined for a certain customer, the invoice due dates are calculated as follows:
If the posting date is between the 1st and the 10th of the month, the due date of the invoice is the 10th of the same month.
If the posting date is between the 10th and the 20th of the month, the due date of the invoice is the 20th of the same month.
If the posting date is after the 21st of the month, the due date of the invoice is the 10th of the next month.
Note
If payment dates are defined in the Business Partner Master Data window, the invoice due date is determined by these payment dates and not by the payment dates defined in the payment terms.