A New Moon on the day of our festival
“Hemdat Yamim”, inside “Days of Awe”, edited by Shmuel Yosef Agnon, page 19:
The first sages have already debated the question why did the Torah concealed Judgement Day and did not explicitly mention it? The main reason is that the Torah concealed the Day of Judgment, so that the person would be doubtful every day that this might be the day would not know the fixed time and would be sinful throughout the year and repent only close to Judgment day, as we always do after Yom Kippur, since every man returns to his evil ways when he thinks he is far from next year’s Judgment Day, this is the temptations of evil inclination ...
As said (Tehilim 81) “at the new moon on the day of our festival”, because the Torah has concealed it, to have them be doubtful every day that this might be the day when God sits to judge the people, and for that reason, one will mind his ways each day.
And when it says: “Sound the ram’s horn at the New Moon, on the day of our festival”, it means on that same holiday which the Torah concealed its time. And why did it conceal it? “because a law for Israel is judgement to the God of Jacob”, which is the day of judgement. That is why the Torah has concealed it, so that they would think every day that it might be Judgement Day.