Rashi on Exodus 19:1/רש"י על שמות י״ט:א׳

1ביום הזה THE SAME (lit., this) DAY — on the day of the New Moon. (Mekhilta; Shabbat 86b). It ought not to write ביום הזה, but ביום ההוא, “on that day”; what, then, is the force of the words “on this day”? Since they refer to the day when the Israelites came to Sinai to receive the Torah they imply that the commands of the Torah should be to you each day as something new (not antiquated and something of which you have become tired), as though He had only given them to you for the first time on the day in question (Yalkut Shimoni on Torah 273; cf. Berakhot 63b).