Or HaChaim on Leviticus 23:27/אור החיים על ויקרא כ״ג:כ״ז

1 יום הבפורים הוא מקרא קדש, "it is a Day of Atonement, a holy convocation." The reason the Torah writes the word הוא is to tell us that this day is in a class by itself, the date alone making it a Day of Atonement independent of any pronouncement. As a result one qualifies for reward by observing the second half of the verse, i.e. proclaiming it holy and afflicting oneself through abstention from food and drink, etc. This is an additional bonus apart from the atonement we are granted simply by being alive on that day.