Sometimes an order arrives over more than one day. This usually happens because of product availability, or because a different delivery day was picked at checkout.
Why this happens
Product availability. If an item isn't available in time, it may come in a later delivery.
The delivery day chosen at checkout. If you add to an order and pick a different day by mistake, those items arrive on that day instead.
Avoid it
Double-check the delivery day when you add to an order.
Check your delivery days
Use the schedule view to see what's arriving and when. See How do I see my upcoming deliveries?