Section 1026.36(c) of Regulation Z states,”no servicer shall fail to credit a payment to the consumer’s loan account as of the date of receipt, except when a delay in crediting does not result in any charge to the consumer or in the reporting of negative information to a consumer reporting agency…” Crediting on Monday, instead of on Saturday, increases the amount of interest that is due, resulting in an additional charge to the consumer. the Commentary to Section 1026.36 states, “The “date of receipt” is the date that the payment instrument or other means of payment reaches the mortgage servicer.”
You need to credit payments on the date of receipt (Saturday) or you must follow the procedures for establishing a payment requirement as explained in the regulation.