Or HaChaim on Deuteronomy 9:3/אור החיים על דברים ט׳:ג׳

1 וידעת היום, "and you know as of this day, etc." Moses emphasises the word "this day" to remind the people that they will not acquire this knowledge only after the conquest but they know as a fact already now that G'd will pass ahead of them, etc. It was important that the people should acknowledge this as fact prior to it actually having happened.