When is the monthly due date to pay for items bought?

That’s too general a question; how did you buy those items? If it was with an Apple Card the payment due date is always the last day of the month. For purchases from the iTunes & App Store, they are billed usually a day or two after the purchase date. For subscriptions the due date is shown when you go to Settings/[your name]/subscriptions and tap on a subscription to see when it will be charged. If you mean something else, please explain.