Or HaChaim on Leviticus 16:2/אור החיים על ויקרא ט״ז:ב׳

1 ויאמר ה׳ אל משה, G'd said to Moses, etc. The Torah employs the "soft" language usually associated with the word אמור, although the subject matter is a dire warning according to Rabbi Eleazar ben Azaryah. The very warning was a demonstration of G'd's loving concern for Aaron, just as the physician who warned the patient of the consequences of not following his advice did so out of concern for the life of his patient.

2Alternatively, G'd demonstrated how much He valued Moses by showing that even when His message concerned primarily Aaron He did not address Aaron with that message but Moses. We may understand the very words ויאמר ה׳ אל משה in a way similar to Deut. 26,18: וה׳ האמירך היום, "and G'd achieved by speaking to you this day, etc." Bamidbar Rabbah 14,21 comments on this that even where we find the Torah reporting that G'd "spoke to Moses and Aaron," the meaning is that Moses was to tell Aaron what G'd had said to him. Our verse then is proof of that statement seeing that G'd addressed Moses even when the commandment He wanted to communicate was addressed exclusively to Aaron.

3ואל יבא, and that he must not enter, etc. In addition to the reason we offered previously for the unusual letter ו at the beginning of a message, we may say that inasmuch as Moses had told Aaron on the very first day of his anointment as High Priest to go ahead and offer his sin-offering on the outer altar (Leviticus 9,7) the meaning was that it was only the altar situated in the courtyard which Aaron had unrestricted access to. Our sages derive this from the principle called לאו הבא מכלל עשה, that the wording of a positive commandment contains within it a negative commandment which was not spelled out by the Torah specifically. The letter ו in the word ואל provides the clue to that negative commandment.

4בכל עת, at all times. The implication is that there are times when Aaron would be permitted to enter the Holy of Holies. Details of this are mentioned later beginning with the words בזאת יבא אהרון אל הקודש. Once every year Aaron was to enter the Holy of Holies. The reason the Torah had to introduce this information by the negative commandment that Aaron was not to enter the Holy of Holies before mentioning the exception, was to tell him that if he refrained from entering the Holy of Holies during the rest of the year he would merit entering the Holy of Holies on the Day of Atonement. If, however, he were to enter the Holy of Holies during any other period he would forfeit the opportunity of entering it on the Day of Atonement at a time when permission had been granted. The Torah chose the expression בכל עת, so as to prohibit entering the Holy of Holies even on the Day of Atonement except in order to perform certain regulated activities, i.e. to burn incense and to splash blood of two sacrifices onto the dividing curtain. When we keep this in mind we derive another meaning from the letter ו at the beginning of the words ואל יבא. The letter relates to an additional warning. Aaron was not only not to enter the Holy of Holies except on the Day of Atonement, but even on that day he was to enter it only for the purpose designated in our chapter. In Torat Kohanim they derive the prohibition of entering the Holy of Holies on the other days from the words אל הקודש. The reason is that the Torah could have written merely the words מבית לפרוכת and I would have known what was meant. The additional words אל הקודש therefore became available for exegesis. Do not concern yourself with the letter ו at the beginning of the word ואל. It is not unusual for the Torah to write such a letter in such a context. We have proved this at length on other occasions.

5כי בענן אראה על הכפורת, "for I will appear in a cloud above the ark-cover." Seeing the Torah had reported G'd's cloud of glory as becoming visible to the entire people in Leviticus 9,23, the Torah had to tell us that there was an even greater proof of G'd's presence in the form of a light of a celestial source above the ark-cover.